The Flood are fictional parasitic alien life forms in the Halo video game series created by Bungie. They are introduced in Halo: Combat Evolved as a second enemy faction alongside the Covenant; they return in sequels Halo 2, Halo 3, Halo Wars, and as a gametype in Halo 4. The Flood are driven by a desire to infect any sentient life they encounter, and are depicted as such a threat that the ancient Forerunners were forced to kill themselves and all other sentient life nearly 100,000 years ago in an effort to starve the Flood to death.
The Flood's design and fiction was spearheaded by Bungie artist Robert McLees, who utilized unused concepts from the earlier Bungie game Marathon 2. The ringworld Halo was stripped of many of its large creatures to make the Flood's appearance more startling. Bungie environment artist Vic DeLeon spent six months of pre-production time refining the Flood's fleshy aesthetic and designing the organic interiors of Flood-infested space ships for Halo 3.
The player's discovery of the Flood in Halo: Combat Evolved is a major plot twist, 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 Flood has varied over the years; while some found the Flood too derivative and a cliché element of science fiction, some others ranked them among the greatest villains of all time.
The Flood were added early in the development of Halo: Combat Evolved, before the game had made its jump from the Macintosh platform to the Xbox. A design for one Flood form appeared as early as 1997. Commenting upon the inception of the Flood, Bungie staff member Chris Butcher noted that "the idea behind the Flood as the forgotten peril that ended a galaxy-spanning empire is a pretty fundamental tenet of good sci-fi. Yeah, and bad sci-fi too." Another inspiration was Christopher Rowley's The Vang series. The early design for the Flood was done by Bungie artist and writer Robert McLees, who considers himself "the architect" of the Flood; the Flood's roots are reflected in concept art of a "fungal zombie" that McLees did for the earlier Bungie game Marathon 2: Durandal. McLees also did all the early concept art for the Flood.