Flood

The Flood, designated as LF.Xx.3273 by the Forerunners, and referred to as the Parasite and the infection by the Covenant, is a species of highly virulent parasitic organisms that reproduce and grow by consuming sentient lifeforms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient lifeforms in the galaxy - including the vast majority of Forerunners - during the Long Night, prompting the activation of the galaxy-sterilizing Halo Array.