The Flood or the Parasite, as they are known to the Covenant, are a species of highly virulent parasitic organisms that can reproduce and grow by consuming sentient life forms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient life in the galaxy, notably the Forerunners, during the 300-year-long Forerunner-Flood war. They are widely considered to be the greatest threat to the whole existence of life, or, more accurately, biodiversity, in the Milky Way galaxy.

Origins Edit

As the Forerunner-Precursor war drew to a close, the few remaining Precursors that were pushed out beyond the edges of the galaxy to Path Kethona sought vengeance against the Forerunners. A few Precursors were spared by the Forerunners, while others became a dust which could regenerate into their past forms. Over time this dust became so corrupted that it could only cause horrific disease and mutation. The Precursors, driven insane by the rebellion of their creations, embraced this corruption and became part of it, becoming the Flood to destroy the Forerunners as a final act of vengeance against them. They sent several ships containing this powder to Forerunner controlled planets.

Background Edit

The Flood are an extremely adaptable, dangerous, and expendable parasitic life form.

They were first encountered in the Milky Way galaxy by an Ancient Human empire 10,000 years prior to the Forerunner-Flood war. The earliest known instance of the highly infectious viral super cell later known as The Flood form to be encountered came in a fine desiccated powder, contained in cylinders.

Early experiments showed the potential for psychotropic effects in some lower levels but not in humans or San'Shyuum. The primary animals affected were popular pets in human societies: the Pheru, lively and gentle creatures found on Faun Hakkor.

Very small quantities of the powder induced changes in the Pheru that improved their domestic behavior. Soon San'Shyuum adopted the custom of taking Pheru as pets. For centuries humans and San'Shyuum bred and powdered these animals without ill consequences. However the powder soon attached itself to key points in the genes of the Pheru and began to change them, which manifested itself as a peculiar growth found on a third of the powdered Pheru.

A kind of loose, soft fur grew between the shoulders of the pets. Other Pheru were soon found grazing on these companions, consuming their fur-and on occasion even consuming the animals themselves. This was odd, as Pheru were naturally herbivores. This seemed to activate some sort of signal for expansion. Within a very short time, the Pheru were producing far less attractive growths. Flexible striped rods sprouted from their heads, which in turn were also consumed by fellow Pheru, causing abortions and unnatural births.

But this was only the surface of the growing infestation. The Flood spread from the Pheru to Humans, specifically through the humans who ate the Pheru as food. These new vectors forced their fellows to consume infected tissue and spread the infection to whatever they touched. In time, whatever they discarded - limbs, tissue - could also spread infection. The Flood continued to mutate and evolve and fought a war with both the Ancient Human Empire and the San'Shyuum.

Unknown to the Forerunners, the Human/San'Shyuum alliance had developed a way to combat and eventually defeat the Flood. By manipulating key strands of the Floods genetic buildup, and infecting a third of the human population with this code, then placing them directly along the pathway of the Flood, the humans were able to alter the resultant Flood, causing it to turn upon itself. As a result, the Flood began to tear itself apart on a galactic level, further spreading the Human/San'shyuum genetic weapon, and causing the Flood to wipe itself out.

Knowing that it faced its own extinction, what remained of the uninfected, stable specimens fled the galaxy aboard a commandeered vessel and remained dormant for thousands of years, biding their time.

The Forerunners learned of the peril and as a precaution designed and subsequently constructed the Halo Network should the threat one day return. As expected, 10,000 years later, the Flood returned and began its conquest of total galactic domination.

After exhausting every other strategic option, the Halo Array was fired, resulting in the elimination of the original source of the Flood infestation and the death of every sentient being in the Galaxy, effectively removing both the source and its entire food supply.

The Flood exists in a fairly stable state in the absence of a Gravemind. They lack the ability to coordinate their efforts, but the Flood will spawn spontaneously and spread by infecting sentient life forms, specifically those that are self-aware and capable of introspection. This stage of a Flood outbreak is known as the Feral Stage.

During its Feral Stage, the Flood is only capable of local coordination through use of pheromone-based communication. Research indicates that the content of said messages are of very limited complexity. Individual Combat and Carrier Forms have access to the skills and memories of their host; however, once an outbreak establishes a viable Gravemind, the Coordinated Stage begins, and it is at this point the Flood becomes truly dangerous. With the formation of a Gravemind, the assimilated intelligence and memories of consumed hosts become part of a collective consciousness, which is capable of strategic thinking and commanding individual Flood forms in an organized manner.

A Gravemind (like the Proto-Gravemind that gave rise to it) is the fusion of pure Flood Super Cells and the complex neuro-system of hundreds or thousands of sentients. The control it exerts over the more mobile forms it creates is likened to an entity's control over its own limbs. Therefore, during the Feral stage, control is localized at the individual level, but once an outbreak enters the Coordinated stage control is assumed by, and broadcast from, the central compound intelligence.

Although little data exists from any adequately controlled experiment, data gathered in the field suggests that if a mobile form is removed from the influence of the Gravemind coordinating its group of origin it will revert to a Feral Stage. Insufficient data exists on whether mobile forms of one Gravemind would be influenced by another Gravemind, though logic dictates this would be the case. However, because individual Graveminds are more or less identical and are all striving for the same goal, it is likely that a distinction would be impossible to detect.

The Bestiarum states that the society of the Flood closely resembles a "utopian socialist ideal as there is no wealth, no poverty, no want, no crime, and no disease; every individual works for the advancement of the Flood as a whole." It also states that it "is unfortunate that their one overriding goal is to [butcher and consume] every other life form in the galaxy."

History Edit

Human-Flood War Edit

The Flood first entered the galaxy from the Large Magellanic Cloud at the edge of the Milky Way roughly 110,000 years ago, contained in cylinders in unmanned vessels where they were kept as a powder. A considerable amount of time passed between their initial discovery by prehistoric humans and their evolution into their current form. The Flood were eventually able to infect Human worlds, sparking the Human-Flood war. The Humans, who were losing worlds to the Flood, continually searched for new territory and went as far as to take Forerunner occupied planets—Or completely glass them if Flood infestation was detected. The Forerunners, seeing this as an act of aggressive imperialism rather than an attempt to hold to the Mantle, retaliated, precipitating the Human-Forerunner war. However, the Humans successfully developed a 'cure' for the Flood infection and were able to eradicate it before falling to the unmolested and more powerful Forerunner military. It would be thousands of years before they were again able to threaten the galaxy.

Unbeknownst to the humans, they had not actually found a cure; the Flood had chosen not to infect the humans instead; The precursors who were now the Flood allowed the idea of a cure to gain credence, so that in the future more efforts would be diverted to attaining it rather than simply excising the infection. When the Flood returned, the Forerunners—Who had devolved humanity and destroyed its civilization—were entirely unprepared to challenge the Flood properly.

Forerunner-Flood War Edit

More than 100,000 years prior to 2552, the Flood encountered a Forerunner research expedition on the planet G 617 g1. Initially, the Forerunners severely underestimated the potential threat this new life form posed, using tactics more suited to disease outbreak than true warfare. By the time the infection had begun to spread, the Flood had continued to grow in biomass and were a deadlier and more intelligent foe. The Flood used unarmed civilian assets to penetrate planetary defense groups, first striking at the Forerunner-held planet of LP 656-38 e, completely bypassing the Forerunner Orbital Fleet and infesting the planet below. They soon moved to similarly infest the planet DM-3-1123b. The sheer numbers of Flood forms on the planets overcame Forerunner ground forces. While at first the Flood demonstrated a lack of even basic group tactics, they had numerical superiority; billions of forms dedicated to the assault, and every member of the Forerunner population a potential Flood host. In desperation, the Forerunner's armada was ordered to immediately commence full planetary bombardment on infested worlds, although at a great cost - many Forerunners were unable to be evacuated before bombardment commenced, leaving the Forerunner military with shallow victories. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers appeared to activate localized weapons of mass destruction, effectively committing mass suicide, but stopping the growing infestation.

Thereafter, the Forerunners realized that ordinary naval tactics were unable to stem the growing tide of the Flood, and decided to pin their hopes of defeating the Flood on super-weaponry. While the Forerunners initially deployed theSentinel robotic drones to fight against and contain the Flood, almost immediately afterward, the Forerunner Fleet Command contemplated "Premature stellar collapse"; using naval battle groups to destroy planetary star systems' primary stars in supernova that would engulf infested worlds and prevent any possibility of Flood infection.

The Flood grew exponentially, spreading from system to system with the intent to infect the Forerunner population rather than wage war with Forerunner battle groups. The Flood had reached sufficient proportions to create 'Key Minds' Enormous instances of Gravemind with a vast computational capacity that challenged even the forerunners most powerful machines. The tide began to turn against the Forerunners; the overwhelming computational power of the Gravemind allowed it to outmaneuver the Forerunner Fleets. The Gravemind convinced the Contender class Artificial intelligence 032 Mendicant Bias to join its cause, along with any other AI it could directly interface with. The Forerunners suspected that the Flood had a logic altering informational virus that was effective against all of their AI.  Mendicant Bias became rampant, and defected to the Flood cause to destroy its former masters. The Forerunners began manipulating Precuror technology—specifically the 'Star Roads' enormous devices that warped spacetime, both destroying Forerunner ships caught in in systems and preventing them from jumping to Slipspace and escape.

The Forerunners began their "Conservation Measure" in force, researching the Flood while constructing an Artifact onEarth and other worlds to generate a Slipspace portal leading to The Ark, which they used to efficiently save as many sentient beings as they can while cataloging and preserving parts of the planet's ecosystems. 

The Forerunners employed many, many measures, including intricate naval tactics in an attempt to contain the Flood, which ultimately failed. Finally, the Forerunners were forced to a Pyrrhic solution, utilizing twelve ring-like super-weapons, known as the Halo Array, across the galaxy (though only Seven survived to their activation) that had been previously built under the machinations of the Master Builder, Faber. Because the Flood were parasitic, and their survival was directly linked to the presence of potential hosts, the Forerunners reasoned that eliminating all potential Flood hosts, that is, all sentient life forms in the galaxy, would render the parasite harmless by way of it having nothing to infect, as explained by the Forerunner AI 343 Guilty Spark. After all countermeasures failed, the Forerunners activated the Halo Array in 97,448 B.C, unleashing galactic destruction, and destroying almost all sentient life forms in the Galaxy. However, the human race and many other species throughout the galaxy had been indexed before the purge by The Librarian. Population samples were preserved on the Ark, or within the Shield Worlds, and out of range of the Halo Array.

The Flood was contained, and unable to reproduce and grow due to lack of hosts, and eventually died out. The only surviving specimens were contained in state of the art, high-security Forerunner research facilities, such as the Halo Installations and the Threshold Gas Mine. After the Flood threat was destroyed, Keyships were dispatched, and reintroduced the indexed species to their origins, sparking life back into the Galaxy again.

Battle of the Apex Edit

Thousands of years after the end of the Forerunner-Flood war, the human ship UNSC Spirit of Fire jumped from Arcadia to follow the signal of Professor Anders, who had been captured by the Covenant. They were led to an unknown planet, which happened to be a Forerunner Shield World. Containment measures on this planet had failed catastrophically, and the entire surface of the planet was choked with Flood biomass. Ground forces were deployed to investigate the presence of unknown hostiles sighted by the ship's radar. After fighting their way through Covenant forces, Sergeant Forge and several Marine squads were attacked by the Flood.

The surviving UNSC forces, which included Sergeant Forge, were ordered to recover some Elephants that were surrounded by Flood forms. After one was rescued, the small squad made their way into a site that would become a base. They then assembled a larger force to rescue the other soldiers. Following their successful rescue, the UNSC forces prepared to engage the Flood.

SPARTAN-II Red Team, while searching for Anders, was ambushed by several Flood forms, which forced their Warthog to crash. The remaining forces at the nearby base, led by Sergeant Forge, moved out to rescue the ill-fated team. As they attempted to rescue the SPARTANs, they soon discovered that the Flood was creating a Proto-Gravemind, and that it may have been jamming the tracking signal that Anders was giving out. On their way to the soon-to-be Gravemind, they encountered several Flood Colony forms, and found that by destroying them, the Proto-Gravemind would become weakened. Once they destroyed the Proto-Gravemind, Serina found a signal, which was believed to be Anders'.

Captain Cutter then ordered all UNSC personnel on the planet to return to the Spirit of Fire. These orders were impeded by the Flood, which attempted to prevent the humans' retreat, but the Flood were eventually held back long enough for the UNSC forces to return to the Spirit. The Flood attacked the Spirit as it entered the interior of the Shield World, but was wiped off by the ship's crew, Sentinels, and the Shield World's cleansing rings. All the Flood forms on the Shield World were destroyed when Sergeant Forge used the Spirit of Fire's Shaw-Fujikawa Translight Engine to destroy the Shield World's internal sun, obliterating all Covenant and Flood forces on the Shield World. Spirit of Fire narrowly escaped by performing a hastily-calculated slingshot maneuver around the exploding sun in order to shoot off into space. Without the Spirit's Slipspace Engine, however, the ship was lost in space, with humanity's only knowledge of the Flood lost with it. However, a number of Flood forms survived the destruction of Shield 0459 by infiltrating Spirit of Fire and had seemingly taken over the ship by 2558.

The Battle of Installation 04 Edit

Although the first firing of the Halo Array starved large numbers of the Flood, samples of parasitic specimens were kept alive (perhaps in stasis) on various Forerunner outposts for research and further scientific study. One such research site was located on Installation 04. This proved to be a grave, irreversible mistake, as both Humanity and the Covenant stumbled upon the abandoned ring thousands of years after the firing of the Array and the destruction of the Forerunner and most of the Flood. Cortana, aboard UNSC Pillar of Autumn, made what was presumed to be a "blind" jump through Slipspace (although it was later revealed that she used translated coordinates from a Forerunner artifact found onSigma Octanus IV) to evade Covenant pursuers. This led both species to Installation 04, and in turn, the Flood that were kept there. After the crash of the Pillar of Autumn on the Halo Ring, the surviving crewmembers dug in for what they believed would be a long and brutal guerrilla campaign against the Covenant forces on Halo.

Meanwhile, the pursuing Covenant cruisers (among them Truth and Reconciliation) amassed their ground forces on the surface, and soon discovered an underground Forerunner Flood Containment Facility in the swamps of Halo. Due to the secure nature of the facility, the Covenant came to use it as a fortified base of operations and an area for storing weaponry, oblivious to its true nature. Though some of the Flood managed to escape, the terrified Covenant soldiers eventually pushed the Flood back in. It is unknown whether a few still escaped.

After Captain Jacob Keyes was captured by the Covenant for interrogation, he caught wind of this weapons cache and, upon his rescue, organized Fire Team Charlie to seize it. When the team arrived at the swamp however, they found the facility was completely devoid of Covenant personnel. However, upon takeoff, a Pelican Dropship, (Call Sign Victor 933) which had transported them to the base, crashed into the swamp as a result of enemy action. Then, immediately after the crash, the pilots were able to record a message about their attackers, calling them "Unknown Hostiles." These "Unknown Hostiles" were actually the Flood, and shortly afterward, they could be seen creeping throughout the swamp by John-117. At first, when he saw the Covenant retreating out of the Forerunner structure running from Assault Rifle fire, he believed them to be allied forces. However, after he opened the door that Captain Keyes had unlocked, he learned the truth through Private Jenkins' recording: this was no Covenant weapons cache. He then proceeded to escape, escorted the surviving marines to a drop zone, and encountered 343 Guilty Spark. Guilty Spark then promptly teleported him into the Library and informed him, "The Flood is spreading. We must hurry!"

The few surviving Covenant soldiers left were scattered and disorganized, and put up little resistance to the new Combat Forms created from numerous Covenant corpses. It is presumed that the Covenant occupants of the research facility had moved through the structure in hopes of finding Forerunner artifacts, and had inadvertently released living Flood specimens. After witnessing the capabilities of the Flood, the Covenant forces locked the facility down and evacuated all personnel who survived the Flood's escape. Keyes and his team were unaware of this fact, however, and continued to delve deeper into the facility, ultimately coming across one of the unopened Flood Containment Chambers. Believing it was a weapons cache of some sort, they unlocked the door, and were soon attacked and assimilated by the Flood inside. With a large supply of "edible" sentient life now occupying the ring, the Flood began to multiply and infest most of Halo. Soon, the infestation proved too great even for Halo's automated military force, the Sentinels, to handle. The ring's commanding AI, 343 Guilty Spark, was forced to call upon a "Reclaimer" (First Staff Sergeant Marvin Mobuto, and subsequently John-117 when Mobuto failed) to activate the array once more. John-117 successfully retrieved the Index, the item required to activate the ring and returned to Halo's Control Center.

These events brought the knowledge of the Flood to the UNSC. In a war of losses, this was another grim revelation for humanity.

After the Flood's escape, the Covenant forces, who were widely stretched across Installation 04, were heavily engaged, and were being overwhelmed by Flood forces. While in some engagements, the Covenant were able to hold the line by using vehicular and air support, most of the weaker Covenant soldiers such as Grunts succumbed easily to the Flood, and the few Elites who remained were outnumbered and destroyed. The UNSC, having a much smaller presence on the Ring, found out about the Flood's might a short while later, when a UNSC ODST force engaged with the Covenant near Alpha Base, was ambushed by the Flood and suffered heavy casualties. However, one of the attacking combat forms, Wallace A. Jenkins, who was one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by a weak and elderly infection form whose powers of infection had been dulled by years of hibernation, making Jenkins' transformation into a combat form incomplete. Jenkins was still able to resist the pervasive Flood entity in his mind, and although incapable of articulating speech properly and with a physiologically devastated body, Jenkins was still able to betray the Flood through hand gestures during an interrogation with Major Antonio Silva, informing the UNSC Marine battalion that the Flood planned to assault the UNSC base from a series of underground catacombs underneath their base. In response to this intelligence, the UNSC moved to secure them, and prevented the Flood from staging another attack.

By the time of these developments, Flood dominance over the Covenant was assured. The Sentinels were unable to contain the Flood, and the Covenant and UNSC were sustaining heavy losses and rapidly losing ground. Meanwhile, the Flood were occupying whole areas formerly held by the Covenant, although Covenant and Flood forces continued to wage war on the massive ice plains of the Ring. Although the Covenant had the benefit of Shade class stationary plasmaturrets and vehicular/aerial support, the Flood had captured both UNSC and Covenant weapons and personnel, most notably M19 Rocket Launchers and massive numbers on their side.

The Flood, however, were not unintelligent. One of the UNSC personnel captured in the Marine force to the ex-Covenant arms cache was Captain Jacob Keyes, a brilliant naval strategist of the UNSC Navy. When he was infected, instead of using him in combat, the Proto-Gravemind, while examining and deleting his memories, felt that Keyes held vital information. Keyes knew the location of Earth, the birthplace and major planet of the human race, a planet with tremendous possible assimilation opportunities for the Flood. Instead of becoming a combat form, he was merged with at least four other victims into a massive, engorged Proto-Gravemind aboard the CCS-class battlecruiser, Truth and Reconciliation, a Covenant warship that the Flood were relentlessly attacking. While he was in the brain form state, the Flood began to comb his memories. Captain Keyes successfully withheld information about Earth by constantly accessing information available on his Command Neural Interface such as his name, rank, and serial number, until theChief arrived. However, his arrival was too late, for Keyes was already dead. It was at that time the SPARTAN-117 punched a hole in Keyes' skull to get the Captain's CNI.

The Covenant Flagship Truth and Reconciliation was a major point of contention between the Covenant Special Operations units and the Flood. The Flood had likely entered the warship previously, decimating most of the Covenant crew and garrison on board, leaving only isolated pockets of resistance, mostly small numbers of terrified Grunts and a few Elites aboard the vessel. However, below Truth and Reconciliation, in a series of rocky canyons, the Covenant were much stronger in force, with large numbers of Elites augmented by Grunts, Jackals, and Hunters, fighting off the Flood. The Covenant High Command on Installation 04, however, were terrified by the Flood presence. Because the CCS-class battlecruiser had been damaged in a previous space battle with UNSC Pillar of Autumn, it had been grounded, awaiting repairs for at least two days. After The Flood were discovered to be aboard the battle cruiser, the Covenant High Command sent a Special Operations strike force to Truth and Reconciliation to commence immediate emergency repairs, prepare it for lift-off into space and neutralize The Flood. Later, the remaining UNSC Marine forces on the ring staged an assault on the Flood-held Truth and Reconciliation, and managed to neutralize most Flood forms on the cruiser in a swift assault, and prepared the battle cruiser for take-off with emergency repairs to escape from the ring to Earth. However, there was tremendous risk if the Flood-infected vessel traveled to Earth, if there was even one Flood carrier form aboard, "Earth could fall," according to UNSC AI Wellsley, because of the Flood's exponential parasitic potential. Against orders, Marine officer McKay destroyed the warship with whatever Flood forms were still aboard, killing hundreds of UNSC personnel, including an infected Jenkins, but saving Earth and possibly billions of lives from the Flood.

The Flood appeared to be interested in seizing space-capable vessels, in the hope of spreading itself away from the Installations and into the stars: they attacked UNSC Pillar of Autumn and CCS Truth and Reconciliation, for this reason. After the UNSC cruiser's crash-landing onto the ring, and after engaging over fifteen enemy warships, it had been taken by the Covenant, although a UNSC raid had managed to temporarily retake the hull. On the third and final day of the Battle of Installation 04, however, the Covenant were in control of the crashed warship when a Covenant patrol was ambushed by the Flood and infected; when it returned into the vessel, the Covenant garrison was attacked by Elite Combat forms. The combat forms were able to scatter before they could all be destroyed, and a steady trickle of Flood continued to enter the Pillar of Autumn through vents and openings in the hull, waging a tedious guerrilla battle against the Covenant security force on board. Eventually, the Sentinels came on board, overwhelming the dwindling Covenant forces as the Flood began to attack the warship in full force, in the end almost completely obliterating the Covenant presence, and severely limiting the Sentinels on board. Despite their best efforts and enormous numbers, the Flood, Covenant, and Sentinels were unable to stop John-117 from detonating the wrecked warship's reactors, and escaping in a C709 Longsword-class Interceptor. The resulting thermonuclear detonation severed the ring world, destroying it and its capability to annihilate all sentient life, and simultaneously killed all remaining Flood on Installation 04, along with any other life forms that were on the ring, other than Zuka 'Zamam and a grunt commander, Yayap, who both escaped on a Spirit.

Threshold Gas Mine Edit

The Flood were also kept in the Forerunner-constructed Threshold Gas Mine, in one of the three peripheral arms of the station. A small Covenant special forces strike team of three Phantoms loaded with Special Operations Elites andUnggoy, led by Special Operations Commander Rtas 'Vadumee and Thel 'Vadamee, were dispatched by the High Prophets to this station. This was to "silence" Sesa 'Refumee, the leader of the Heretics stationed within the mine encouraging dissent against the Prophets. While the force attempted to track down Sesa 'Refumee to assassinate him and destroy his forces, the Heretics unleashed the Flood that were held within the station.

During Thel 'Vadamee's pursuit of Sesa 'Refumee, the strike team entered a laboratory, in which they were locked into while the Flood was released in an attempt to quell the attackers. The plan failed, and instead allowed the Flood to infiltrate other parts of the station, killing both Covenant and Heretics alike. The remaining Covenant strike team then continued through a large elevator that ran in the spine of the Flood-infested arm. While presumably used to transport gas conduits, its walls were rampant with Flood, and it ferried canisters full of Infection Forms. The tenuous, agonizing elevator was nicknamed "A Descent Into Hell." The Flood began making its way up the station, overtaking the Heretic forces and forcing 'Refumee to seal himself inside another section of the station. This action forced Thel 'Vadamee to sever the cables connecting the station to the higher atmosphere. 343 Guilty Spark later explains this was a strategic placement by the Forerunners, as afterwards the facility would fall into the planet's solid Nitrogen core.

Battle of Installation 05 Edit

Unlike on Installation 04, where the Sentinels had contained the Flood until the Covenant began to meddle with the Forerunner containment measures, on Installation 05, the Flood had been in continual war with the Sentinels since the firing of the Halos 100,000 years ago. Even up to 2552, the Flood were presumably dwindling with no fresh supply of hosts (perhaps they were involved more in hiding than in open warfare). However, like on Installation 04, the fresh suitable host bodies of a newly arrived Covenant armada and a UNSC strike force fueled the Flood's exponential parasitic capabilities, and Flood infestation increased at a surprisingly rapid rate, and within hours, Sentinel forces were being overrun, causing a risk that the Ring would fall to the parasite.

In order for the activation of the Halos and the facilitation of the ultimate Flood containment measure to occur, a Reclaimer had to retrieve the Index from Installation 05's own Library, making the Library a high-priority target for the Flood: if it could control the Library and prevent any Reclaimer from entering, it would make itself essentially invulnerable. Installation 05's Library was nestled in a tenuous series of wintry and snowy canyons and boasted three levels of defense: an exterior Sentinel Wall kilometers in the distance, an underground honeycombed facility that produced large numbers of Sentinels (that impeded any invaders from advancing), a Containment Shield (an energy shield that denied access by any means to the Library until it was deactivated), and a Quarantine Zone (a type of biological "airlock" to prevent the Flood from spreading any further). This multi-tiered defense was patrolled by hundreds of Sentinels and Sentinel Majors, and several Enforcers.

However, two external forces, the Covenant and the UNSC, wanted the Index as well: the Covenant, to activate the Halos and set off the misinterpreted "sublimation event" of the Great Journey, and the UNSC to secure the Index and prevent the Covenant from doing so. The Covenant sent a Strike Team of Special Operations Elites, commanded by Rtas 'Vadumee, and led into battle by Thel 'Vadamee, to retrieve the Index in a multi-staged assault. However, the Covenant first needed to deactivate the Containment Shield by disconnecting its power generator in the center of the Sentinel Wall.

Previously, a force of Jiralhanaes, Unggoys, and Kig-Yars had been dispatched to complete that operation, but they were overwhelmed by Sentinel defenders and forced into a defensive position instead of an attacking one. Thel 'Vadamee was personally ordered by the Prophet of Truth to find and retrieve the "Sacred Icon" (the Covenant term for the "Index") for the Covenant. Because the Sentinels activated the Containment Shield, he could not be directly air-dropped to the Library; instead, he had to land at the periphery of the Sentinel Wall, disable the generator, and then proceed to deactivate the generator. Tartarus deployed Thel 'Vadamee in a Phantom drop ship, where Thel 'Vadamee entered the first half of the Sentinel Wall, where there was evidence of heavy Covenant casualties and abandoned infrastructure, making his way down through the facility and finally deactivating the Containment Shield.

While the first half of the Sentinel Wall had remained pristine, the second half, the one buttressing the Quarantine Zone, was infested by Flood. Sentinel resistance had been destroyed in some segments of the Flood-Infested Wall,[9] and in heavily infested areas, the Flood appeared to be beginning to terraform the Forerunner facility, spreading murky spores into the air. The UNSC deployed a small number of Marines to fight through the Flood-Infested Wall, but they were quickly overrun. Thel 'Vadamee was able to battle through hundreds of Flood forms that had taken up positions in the Wall, which had entered through wall-mounted conduits, although later, the Covenant deployed a team of three Special Operations Sangheili in the lower levels of the Flood-infested Wall, although they were killed as well. Finally, Thel 'Vadamee was able to make his way out of the Wall, emerging at the ground levels: the Quarantine Zone.

With the Sentinel Wall compromised by the Covenant and the Flood, and the Containment Shield lowered by Thel 'Vadamee, there was only one last line of Forerunner defenses: the Quarantine Zone. There was a significantly elevated number of Enforcers present, even automated sentinel construction factories flying over watch, and automated factories built into the landscape, each churning out Enforcers and Sentinels, further swelling their numbers even in intense combat. Here, the Flood demonstrated their unusual technical aptitude, commandeering Covenant and UNSC vehicles, possessing dozens of Ghosts, Wraiths, Warthogs, and Scorpions to do battle with the hovering Enforcers. Flood forces also boasted large numbers of Combat Forms, which appeared to infiltrate the Containment Zone by means of gaping chasms that periodically split the ground, emitting a green mist and appearing to be of indefinite depth, perhaps linkages to the Flood-infested underground.

Six Special Operations Elites, along with Rtas 'Vadumee, were airdropped at the periphery of the Quarantine Zone by means of Covenant drop pods, and Thel 'Vadamee joined them, rallying their efforts to secure a landing zone against large numbers of Flood attackers. Using the advantages of elevated ground, Shielded Plasma Cannons, and aDeployable lookout tower, the small Special Operations contingent was able to hold an LZ for a Phantom to drop off aSpectre and a pair of Ghosts. Meanwhile, 'Vadumee commanded his men to assist Thel in forging towards the center of the Flood-infested Quarantine Zone, so that Thel 'Vadamee could reach the Library and secure the Sacred Icon.

The Flood, despite a significant vehicular and infantry presence in the Quarantine Zone surrounding the Library, were unable to deny access to the Covenant and the UNSC. Miranda Keyes managed to slip the In Amber Clad through the Forerunner Enforcer patrols, while the Covenant launched a ground borne attack, led by Thel 'Vadamee and involving a small team of Spec Ops Elites, using a gondola to get to the index while attempting to control the gondola and keep it from slipping into Flood or Sentinel hands. Thel 'Vadamee took the Index from Miranda Keyes and Sergeant Johnson, who had gotten there first, but was betrayed by the Brutes and knocked into the depths of the Library. Later, In Amber Clad was captured by the Flood.

Attack on High Charity Edit

John-117, who had arrived with Keyes and Johnson, met Thel 'Vadamee in the clutches of the Gravemind, the central controlling intelligence of the Flood hive, created from the combined intellect of tens of thousands of assimilated corpses. The Gravemind had saved Thel 'Vadamee before the Elite could fall to his death after being hit by a burst from Tartarus' Gravity Hammer. Gravemind also managed to capture John-117 as he sank in a lake. Gravemind attempted to convince Thel 'Vadamee, against his own beliefs, and through the use of the captured Monitor, 2401 Penitent Tangent, and the semi-revived Prophet of Regret to not activate the ring. Still unswayed, although in question of his religion, Thel 'Vadamee prompted the Gravemind to show him, by "stopping the key from turning," transporting Spartan-117 to High Charity, and Thel 'Vadamee to Delta Halo's Control Room.

Taking advantage of the distraction that Spartan-117's presence caused, as well as the ensuing Covenant Civil War, Gravemind took the opportunity to deliver himself, as well as a multitude of Flood underlings, onto In Amber Clad and then jumped the UNSC ship inside High Charity, whereupon it immediately crashed into the city's superstructure to deliver its infectious cargo, while a wave of Flood-controlled Pelican Dropships delivered Flood all over the city. The ill-equipped and distracted populace was no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the Prophet of Mercy falling victim to an infection form.

The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems; the whole city was becoming Terra-formed into a massive Flood Hive. The Gravemind regularly rebutted the words of comfort the Prophet of Truth delivered to the city’s population over the comm system, to sadistically instill more panic and unease. Spartan-117, having successfully stayed in front of the wave of Flood running rampant in the streets, stowed away on the Dreadnought that was leading the Prophet of Truth's fleet to Earth. Cortana, who had to stay behind to manage the situation should Installation 05 be fired, was left in the clutches of the Gravemind, who could potentially learn all possible Human knowledge from her.

Earth, the Ark, and the Flood's Defeat Edit

At the conclusion of the Battle of Voi, with the Prophet of Truth activating the Artifact and the Jiralhanae fleet escaping to the Ark, a Flood-infested Covenant cruiser appeared out of Slipspace, having penetrated the Elites' blockade of High Charity and crash-landed into the city of Voi, attempting to spread. In order to prevent the Flood from escaping and infecting the entire planet, Miranda Keyes ordered John-117 to detonate the cruiser's reactors and search the ship for Cortana, destroying the city of Voi and eliminating the present Flood. As John-117 and Thel 'Vadam fought their way to the ship, the Shadow of Intent arrived and deployed Sangheili task forces to aid UNSC marines. SPARTAN-117, with the help of the Sangheili, was able to battle through the Flood forces, including the newly encountered Flood Pure Forms, to reach the crashed cruiser, where he found a message from Cortana. Upon this discovery, 343 Guilty Spark arrived pledging his assistance to the Reclaimers. With the Flood spreading, SPARTAN-117 boarded a Phantom while the Sangheili went on to glass the infected areas. The remaining Sangheili forces, fulfilling their mission of removing the Flood from Earth, retreated back to the Sangheili Carrier, the Shadow of Intent, along with Cortana's message and Guilty Spark.

With the assistance of 343 Guilty Spark, Cortana's message was repaired, revealing a solution to the Flood menace on the Ark. To Lord Hood's disapproval, Miranda Keyes led UNSC forces allied with Thel 'Vadam's and Rtas 'Vadum's Sangheili through the Portal.

On the Ark, the UNSC and the Covenant Separatists attempted to get to the Prophet of Truth, but first had to disable the shield over the Ark's citadel by attacking three towers funneling energy to the shield. John-117 led the attack on the first tower, while Thel 'Vadam led the attack on the second tower, and Sergeant Johnson assaulted the third tower. When Sergeant Johnson failed to take out the third tower, John-117 arrived there with Thel 'Vadam and a group of Elites. John went to the top of the tower. While attempting to lower the shield surrounding the Ark's citadel, the infested High Charitymade a Slipspace jump and smashed into the Ark, releasing the Flood inside the structure and causing an infestation on the newly built Installation 04B. All three factions were immediately attacked by Flood forces. Significant damage was done to the Shadow of Intent, making the ship virtually inoperable. The UNSC and the Covenant Separatists were forced to attack Truth on the ground. Finally, after attacking two Scarabs protecting the Citadel, the two forces managed to secure access to the Citadel. Spartan-117, followed by Thel 'Vadam, quickly entered the Citadel. Inside, Truth, along with several Jiralhanae, was attempting to convince Sergeant Johnson to activate the rings. Miranda Keyes desperately crashed a Pelican into the Citadel to stop Truth from activating them, but was ultimately killed by Truth. Truth then forced Johnson to activate the rings. The Gravemind realized that if the rings were to fire, it would be destroyed along with its loyal minions.

When it realized that John-117 and the Arbiter were inside the Citadel, it realized that if it helped them kill Truth, the rings would stop charging. It quickly arranged for two Pure forms to meet John-117 and the Arbiter and explain it's request. John and the Arbiter accepted the Gravemind's help and the two attacked the Covenant forces while the Flood assisted them. Finally, they came to the end of the hall. Johnson was holding Miranda Keyes, even though it was too late, and told the two to kill Truth and stop the ring's firing. Thel 'Vadam managed to capture Truth while John-117 deactivated the rings. While Truth attempted to convince the Arbiter to help him, the Gravemind managed to infect Truth, corrupting him from the inside. Thel 'Vadam thrust his energy sword into the Prophet, killing him. The Gravemind then betrayed the two, attacking them with its Flood forces. However, Sentinels managed to subdue some of Flood, while John-117 and Thel 'Vadam made their way through. They escaped through a small hole at the end of the hallway. There, the two saw the newly finished Installation 04B. John-117 and Thel 'Vadam formulated a plan, a plan that, if it worked, would mean the end of the Flood.

John-117 needed Cortana to execute this plan, however, so he took a Banshee and flew straight to High Charity, which had crashed on the Ark. Meanwhile, Sergeant Johnson and Thel 'Vadam began evacuating their troops from the Ark in order to escape the Flood. John-117 managed to enter High Charity and fight his way through. The Gravemind was worried, but believed its Flood forms would be enough to stop the Spartan. However, John pushed ever deeper through High Charity. The Gravemind, in an attempt to stem the Spartan's progress, started torturing John-117 with telepathic messages, all while Cortana was contacting John, nearing rampancy. John-117 continued to push into High Charity, finally reaching the reactor room, where he fought through into the Sanctum of the Hierarchs. The Gravemind then realized what John was trying to do, but it was too late. John-117 rescued Cortana and managed to overload the reactors by destroying three connecting pylons. As he escaped High Charity, the Gravemind started going mad. It sent Combat forms to stop the Spartan, but John fought through them all. He raced back to find Thel 'Vadam attacking Flood Pure forms. The two quickly boarded a nearby Pelican that crashed into the tower. When Cortana appeared on the holopad, Master Chief fired the engines and Thel 'Vadam jumped in. As the Pelican took off, the Gravemind attempted to pull the Pelican back using one of its tentacles, but was burned by the jet wash. The Pelican raced off as High Charity collapsed behind them. Nevertheless, the Gravemind managed to escape, along with a few Flood forms.

As John-117 raced toward the unfinished Halo, Sergeant Johnson assumed command of the UNSC frigate Forward Unto Dawn. Sergeant Johnson landed the frigate as close as possible to the Control Room. Meanwhile, John-117's Pelican crashed into the snowy canyon, the place where the Control Room could be found. As the four make their way to the Control Room, the Gravemind quickly sent its forces to the Control Room. Then, it contacted John-117 once again. The Gravemind sent its forces to begin occupying the new Halo construct. However, 343 Guilty Spark arrived shortly after. He assumed command over the Sentinels and assisted John-117 and Thel 'Vadam as they climbed to the Control Room. As they went up, Sergeant Johnson supported them with his Spartan Laser. Eventually, they reached the top and Guilty Spark opened the door for them. As they hurried to the Control Room, the Gravemind began talking to John-117 again, and Cortana realized that the Gravemind was trying to rebuild its damaged form. As the four went in, the Gravemind tried to follow them with Combat forms, but was locked out. However, the Gravemind was able to regroup when 343 Guilty Spark went rampant following Johnson's attempt to activate the Halo ring prematurely, mortally wounding Johnson and locking the Arbiter out of the room. Guilty Spark then did battle with John-117, using a powerful hard-light beam that easily drained John's shields. Johnson, still alive, shot Spark with his Spartan Laser, before handing it to John while the Monitor recovered from the unexpected attack. Finally, John destroyed Guilty Spark and activated Halo using the Index that Cortana saved from the first Installation 04. The Gravemind, realizing that there was very little chance of survival, attempted to stop John and Thel 'Vadam from escaping Halo. However, the Gravemind's forces were still battling Sentinels, leaving few Combat Forms to stop the two as they took Johnson's Warthog. Despite the Flood and Sentinal forces that stood in their way, John-117 and Thel 'Vadam were still able to make it onto UNSC frigate Forward Unto Dawn. As they flew into the Portal, Halo activated, killing the Gravemind and wiping out all surviving Flood on the Ark and on Installation 04B.

Return on Trost Edit

Despite many defeats, the Flood had one more fight left. Thel 'Vadam, Rhas 'Churol and 'Ensis Urdenee were deployed to investigate a Sangheili village that had been very inactive in the recent months. Anthony Mendez also follows them. The group finds the village, which had been destroyed, and all Sangheili had been murdered. A young Sangheili is found, who describes what happened to the team. Before they leave, however, they find a small infestation of flood. The group retreats after being overwhelmed. A few days afterwards, Agent Mars, Sdan 'Roluk, and Chad-119 deploy with some Elites to the Bifrost Mountains to check on a small human village. After dropping, they move ahead in the snow. The group discovers a pair of children and escort them as contacts appear on radar. They find the village, destroyed like the Sangheili village. Combat forms quickly engage the team. They evacuate, but not before Sdan is hit by a sniper bullet. Afterwards, the UNSC decides it's time to engage. Fireteams Blue, Aquila, Foxtrot, and Eclipse prepare with John-A222 at Firebase Thermopylae in the Tengil Valley of the Bifrost Mountains. The teams travel to Midgard Mountain and enter The Barrow to investigate, recover, and possibly destroy the facility. Inside, they engage Flood forms in the misty dark halls. The teams split up and prepare a HAVOK nuke to destroy the base. Commander Rower orders everyone to evacuate the base as she primes the bomb. A222 drives a warthog to pick Rower up, but there is no response from her and retreats to Firebase Thermopylae. The HAVOK detonates afterwards, but the Gravemind continues its chaos. Flood spores later destroy Firebase Thermopylae, and the spartan squads escape on Pelicans. Remnant's Fall glasses the area, with the help of Normandy, which fires a MAC round. Later, in the city of Serath, many UNSC soldiers defend the city against the onslaught of the flood. Sean Moorland and Ouya Frahman plant a bomb in the city's reactor. when Ouya tries to detonate it, Sean kills her and breaks the bomb in the process. Sean later escapes after helping Marc Bedragare in a hospital with Jackson Forge and John-A222. A hive is later found, where Jack-082 and teams Red and Boltshock plant a FURY nuke in the hive. More Spartans plant another Nuke, and also find a Proto-Gravemind in the process. Joseph Murdoc also loses his arm during the mission. The nuke detonates and destroys the Proto-Gravemind.

Later, a Halo is discovered, Installation 3. The UNSC and Sangheili Separatists go to the ring, where John-A222 and Marc both attempt to secure the index.