Flood

The Flood is an alien parasite, possibly of extragalactic origin. The Flood has been present in the Milky Way galaxy for at least 100,000 years, when they fought a destructive war with an advanced ancient alien race known as "the Forerunners". The Forerunners activated a series of weapons of mass destruction on a galactic scale located on ring-shaped artificial worlds that would later become known as "Halos", destroying both the Flood, themselves, and any other sentient life in the galaxy, save for those species kept in an extragalactic artificial world known as "The Ark". The Flood would be unleashed again when both the United Nations Space Command and their enemies, a coalition of alien races known as the Covenant Empire stumbled across one of the rings. The Covenant unintentionally released Flood specimens kept in stasis on the ring, which quickly killed or assimilated most of the UNSC and Covenant troops on "Halo". The Flood threat was destroyed when UNSC SPARTAN John-117 destroyed the Halo ring by detonating the fusion reactors of a destroyed UNSC starship. The Flood would later be unleashed again on a second Halo ring and "The Ark", this time in a more advanced state led by a hivemind known as "The Gravemind". The Flood was finally defeated when John-117 destroyed the Flood-infested "Ark".