The Flood

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.

Return on Trost
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.