Flood(Halo)

The Flood are fictional parasitic alien life forms in the Halo video game series. They are introduced in Halo: Combat Evolved as a secondary enemy to the game's protagonist, Master Chief, and return in Halo 2 and Halo 3 to fill the same role. The Flood are driven by a desire to infect sentient life they encounter, and are depicted as such a threat that the ancient Forerunner were forced to kill most of themselves and all other sentient life nearly 100,000 years before the beginning of Halo in an effort to starve the Flood to death.
The player's discovery of the Flood in Halo: Combat Evolved is a major plot twist that is kept suspenseful and tense, and was one of the surprises reviewers noted positively upon release. The Flood's return in Halo 2 and Halo 3 was less enthusiastically praised. Reaction to the faction varied; while some publications found the Flood too derivative and a clichéd element of science fiction, Wizard Magazine rated them among the greatest villains of all time.