The Flood are fictional parasitic alien life forms in Bungie's Halo video game series. They are introduced in Halo: Combat Evolved as a second enemy faction alongside the Covenant, and return in Halo 2 and Halo 3 to fill the same role. 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 are forced to kill 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 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-based 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 publications such as The Dallas Morning News found the Flood too derivative and a cliché element of science fiction, Wizard Magazine and PC World Magazine rated 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 Microsoft Xbox. A design for one Flood form appeared as early as 1997. 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. Based on the behavior of viruses and certain bacteria, the Flood were intended to be "disgusting and nasty".
At one point, the ringworld Halo featured dinosaur-like terrestrial creatures, but due to gameplay constraints, these were dropped. An additional consideration was that Bungie felt the presence of other native species would dilute the impact and surprise of the Flood. 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.
For Halo 3, it was decided a new visual language for the Flood was needed. The task of developing the new Flood forms, organic Flood terrain, and other miscellaneous changes fell to Vic DeLeon, Bungie's Senior Environment Artist. Flood-infested structures were designed as angular to counterbalance Flood biomass, as well as provide surfaces for the game's artificial intelligence to exploit and move on. New additions were designed to be multi-purpose; exploding "growth pods" that spew Flood forms were added to the game to adjust pacing, provide instant action, and add to the visuals. Endoscopic pictures provided further inspiration. Bungie used Halo 3s improved capacity for graphics to make a host's sudden transformation into Flood form more dramatic; two different character models and skeletons were fused and swapped in real-time.
The Flood are depicted as having a complicated lifecycle. The largest self-contained form that the Flood can produce itself, without using other biomasses, is an "infection form". The infection form homes in on hosts (living or dead), attempting to drive sharp spines into the host and tap into the nervous system. The host is incapacitated while the infection form burrows into the host's body and begins the mutation process, bringing the host under Flood control. Depending on the size or condition of the body, the Infection form mutates the hapless host into various specialized forms in the continual drive for more food. Larger hosts are seen turned into forms for combat, growing long whiplike tentacles, while mangled and disused hosts are turned into incubators for more infection forms. The Flood also create forms not suited to combat, which interrogate and strip information from the minds of its victims or serve as a central intelligence to drive the infestation.
The Flood reappear in the Halo 3 mission "Floodgate", on board a damaged ship that escapes the quarantine around Delta Halo. While the infestation of Earth is prevented by vitrification of half the African continent, Gravemind follows the Master Chief and his allies to the Ark aboard the infested High Charity. Though the Master Chief and Arbiter form a tenuous alliance with the Flood to stop the activation of all the Halo rings, as soon as the firing sequence is stopped Gravemind turns on the humans and allied Covenant again. The Master Chief fights his way to the center of High Charity, freeing Cortana and destroying the city, but Gravemind attempts to rebuild himself on the ring under construction by the Ark. Realizing that lighting the ring will destroy only the local Flood infestation and spare the galaxy, the Master Chief, Arbiter, Cortana and Sergeant Johnson proceed to Halo's control room, activate the ring, and escape. Gravemind leaves the Master Chief and the Arbiter with a final warning that his defeat will only delay the Flood's goal of consuming all sentient life. Halo 3 added new capabilities to the Flood, including the ability for the parasite to infect enemies in real time. The Flood are also seen to produce a mutable "pure form" which can mutate into several other Flood types.
Despite the positive acclaim in Halo, the response to the presence of the Flood in Halo 2 and Halo 3 was mixed. A panel of online reviewers noted that the Flood appeared in Halo 2 for no obvious reasons, and were simply described as "aggravating" to play against. Similarly, reviewers including Victor Godinez of The Dallas Morning News felt that the Flood were too derivative of other sci-fi stereotypes, and functioned as "space zombies". Daniel Weissenberger of Gamecritics.com noted in his review of Halo 3 that even though the Flood looked better than ever, their single strategy of rushing the player proved tedious over time.
Lee Hammock, writer of the Halo Graphic Novel story The Last Voyage of the Infinite Succor, described the basis of the story as a way to showcase the true danger of the Flood as an intelligent menace, rather than something the player encounters and shoots. Hammock also stated that the story would prove the intelligent nature of the Flood, and "hopefully euthanize the idea that they are just space zombies"; this treatment was received positively by critics. In 2006, Wizard Magazine ranked the Flood as the 77th Greatest Villains of all time, Game Daily ranked them 5th of their "Top 25 Enemies of All Time" and PC World ranked the Flood the 31st "Most Diabolical Video-Game Villains of All Time", while Electronic Gaming Monthly ranked the Flood in their top ten list of "badass undead".More recently, MTV ranked Flood possession in Halo 3 as a "great gaming moment", stating that "with the power of the Xbox 360's graphics, this reanimation comes to vivid, distressing life, more memorably than it had in the earlier games. Here are the zombies of gaming doing what they do worst. [...] It's grisly and unforgettable."