The Flood (Latin Name: Inferi redivivus, meaning "The Dead Reincarnated") is a secondary enemy faction that appeared in the Floodgate set and in the Flood Drop Pod.
The Flood is an extragalactic lifeform that infects and assimilates entire species for its own needs. The Flood is controlled by a single leader, the Gravemind; a massive, hyperintelligent assimilation of billions of dead beings, it is this trait which makes the flood harder to contain. This containment issue meant that the only effective way to deal with the flood was activating 7 halo rings as the forerunners did so at the cost of life itself. In 2552 John 117 would again activate Halo, this time only activating one ring. This ring was however isolated and far from life, the activation killed all local flood, while other forces including John 117 himself managed to escape through a portal.