The Flood

The Flood are one of the two main villain groups in the Halo video game series, the other being the Covenant. They are an ancient alien parasite capable of assimilating any other organic life-form and turning them into flood zombies, and if left unchecked could consume all other sentient life in the universe. Thousands of years ago the Forerunner race were at war with the Flood and created the Halo space installations as a last resort to kill all other life in the galaxy so the Flood would starve to death. It is the goal of the Covenant to activate the Halos, which they mistakenly believe will send them on a "Great Journey" to godhood.

The Flood are all controlled by an intelligent hive mind called the Gravemind, who often speaks through his Flood zombies. Gravemind and the Flood are seemingly destroyed once and for all at the end of Halo 3, but they could return again eventually.