

The Flood is a great enemy type because it can be any enemy type. It continued to grow until it became the first Gravemind. The mass began compounding into a larger mass This mass eventually became large enough to develop sentience. Something went wrong though after the event and their physical mass mutated. They used their advanced minds to remove their essence to another plane and dissipate their bodies into basic compounds (like a mist of organic material) so that one day they could return and reform themselves.

They were overrun and were forced to retreat. Having no notion of war they were blindsided. In the end the Precursors chose humans to take their place which enraged the Forerunners who then attacked the Precursors. The Forerunners were the most advanced race, had lifespans lasting millennia, but Humanity was accelerating at a faster pace despite having very short lives. After millions of years, they had discovered a way to transcend mortality and were ready to leave the 'Mantle of Responsibility' of tending and nurturing the galaxy to a new race. They are what remains of the Precursors, which is the race that seeded the galaxy and devoted itself to growing the species it had brought into being. The Flood are still on the Ark and on several Forerunner constructs that were immune to the firing of the Halo rings. The Flood was never destroyed and quite possible cant' be fully eradicated as even a spore can re-ignite the entire infestation. You faced them in Halo Wars 2 and will deal with them in Halo Infinite. When the Elites were betrayed by the Covenant many of them joined the Banished, along with the Unggoy, Jackal races, and Lekgolo. Following his lead several Brutes splintered off from the Covenant, built up a mercenary force, and further expanded to become more powerful than even the Covenant could handle since by that time they were in a full-scale war with Humanity. They formed when Atriox turned on the Covenant. The Banished were formed before the events of Reach. The last holdout of his faction was defeated on the Sangheili home world by the Arbiter's forces with the aid of Spartan IVs.

They were the faction you fought in Halo 4 and the beginning of Halo 5. He discovered Requiem and believed it contained one of the Forerunners. Many still believed the Forerunners were gods and started a faction to serve the Didact under Jul 'Mdama who was known as The Didact's Hand. When the Covenant lost High Charity to the Flood the Covenant were fractured. The war didn't just end after Halo 3, the Covenant still had thousands of ships even after the schism created when the Sangheili were betrayed and hunted by the Covenant-loyal Brutes. The actual Covenant is gone and the Flood is a great enemy to continue the series with as they part of the origin story for the entire Halo universe.
