The Flood (Latin Inferi redivivus, meaning "the dead reincarnated") 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. The Flood presents the most variable faction in the trilogy, as it can infect and mutate Humans and Covenant species, such as Sangheili, and Jiralhanae, into Combat Forms. 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 Of The Flood
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 diseases. The Precursors saw a chance for revenge and so they created 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.
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 both the Human-Flood war and, as a consequence, the Human-Forerunner war. However, the Humans successfully developed a 'cure' for the Flood infection and were able to eradicate it. 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, but the Flood had chosen not to infect the humans instead; the Precursors who both birthed and were the flood Instead aimed for a grander, much more terrible design than merely mindlessly spreading as a disease. When they returned, the Forerunners who had devolved the remains of humanity were entirely unprepared to challenge the Flood properly.
More than 100,000 years prior to 2552, the Flood encountered the Forerunners on the planet G 617 g1. Initially, the Forerunners severely underestimated the potential threat this new life form posed, and used tactics more suited to disease outbreak than actual warfare. By the time the infection had begun to spread, the Flood had become a much 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, 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 against the Flood. 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.
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 Fortress Worlds or 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," 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.
The Flood is the 'pirate' faction of Sins of the Prophets. They are a non-playable faction and they infect everything and anything that they happen to come along, which improves and increase the number of units it has to throw at you. They can, and will, take planets, ships and even more with ease, capable of taking on your fleets with a suicidal blind assault. Every single vessel that you make is a possible host for a Flood spore.
It is recommended that you build a number of Anti-Fighter Frigates so you can combat the Flood Spores and ensure that you main force doesn't fall victim to infection and destruction. UNSC Players are recommended to stay well away from Flood Planets without a fleet that is capable of dealing with large numbers of Guardian Vessels and Flood Spores. Covenant Players that have Salvation-Class Capital Vessel will find that they are immune to the infection and therefore have a better chance of conquering the Flood and wiping them out completely.