The Flood

The Flood are a species of highly virulent parasitic organisms in the Halo Universe that reproduce and grow by consuming sentient life forms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient life in the galaxy, notably the Forerunners, during the 300-year-long Forerunner-Flood war. They are widely considered to be the greatest threat to the whole existence of life, or more accurately, biodiversity, in the Milky Way galaxy.