Flood (Halo)

The Flood (Latin Inferi redivivus, meaning "the dead reincarnated") or the Parasite, as they are known to the Covenant, are a species of highly virulent parasitic organisms that can 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. The Flood presents the most variable faction in the trilogy, as it can infect and mutate Humans and Covenant species, such as Sangheili and Jiralhanae, into Combat Forms. They are widely considered to be the greatest threat to the whole existence of life, or more accurately, biodiversity, in the Milky Way galaxy.

In When the Cold Breeze Blows Away, it will be first appeared that they are appeared as neutral archenemy to each species or survivors but also appeared in somewhere universes if not only located in Half-Life universe.

They are neutral enemies due to their parasitics which attack anyone or turn anyone into the zombified-like Flood Combat Form unless survivors try attack and kill them.