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, and return in sequels Halo 2, Halo 3 and Halo Wars. 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.
Game development 
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.
Based on the behavior of viruses and certain bacteria, the Flood were intended to be "disgusting and nasty". The creatures were constructed from the corpses and bodies of former combatants, so the artists had to make sure the Flood soldiers were recognizable enough while changing their silhouette enough to differentiate them from the uninfected. Many concepts and ideas were discarded due to time constraints—initially, the Flood were intended to convert any species of the alien Covenant into soldiers. "We didn't have the resources to make it happen," McLees recalled, so they modified the game's fiction to suggest that some Covenant were too small or too frail to be combat troops. At one point, the ringworld Halo featured dinosaur-like terrestrial creatures, but these were dropped due to gameplay constraints. An additional consideration was that Bungie felt the presence of other native species would dilute the impact and surprise of the Flood.
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. With the addition of mutable forms in Halo 3, the only fictional constraint on the designer's imaginations was that the Flood altered the DNA of its victims by digesting.
Early concepts of what became "pure forms" featured the creatures wielding an array of weapons via tendrils, while forms like the Flood Infector and Flood Transport concepts never made it into the final game. 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 3's 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.
Halo: Combat Evolved 
The Flood make their surprise appearance more than halfway through Halo: Combat Evolved's campaign, during the mission "343 Guilty Spark". The Master Chief is sent on an extraction mission by the artificial intelligence Cortana to find Captain Jacob Keyes, who disappears in a swamp while looking for a weapons cache with which to fight the alien Covenant. The Master Chief discovers that the Covenant have released the Flood accidentally, and the sheer numbers of the parasite overwhelm Keyes and his squad. Keyes' squad are turned into soldiers for the parasite, while Keyes is interrogated by the Flood in an attempt to learn the location of Earth. Keyes successfully resists, but is assimilated by the Flood before Master Chief can rescue him. The emergence of the Flood on the ringworld Halo prompts Halo's resident artificial intelligence 343 Guilty Spark to enlist the help of the Master Chief in activating Halo's defenses and preventing a Flood outbreak. When Master Chief learns of the devastation that Halo would cause to humanity and all other sentient life if activated, he detonates the human ship Pillar of Autumn's engines, destroying the ring and preventing the Flood from escaping.
The Flood are depicted as having a complicated lifecycle. The largest self-contained form that the Flood can produce itself, without using other biomass, is an "infection form". The infection form seeks 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 Halo Graphic Novel 
The 2006 book The Halo Graphic Novel expands upon the Flood's release from Halo in two stories. The first, Last Voyage of the Infinite Succor, takes place at the same time as the Master Chief hunts for Keyes during Halo: Combat Evolved. The Flood manage to pilot a Covenant dropship off Halo, and crash-land the vessel in the hangar of a Covenant agricultural ship, Infinite Succor. Successfully assimilating most of the Covenant and wildlife aboard the ship, the Flood are stopped by a Covenant strike team led by Rtas 'Vadumee, who sets the ship on a course into the nearby sun. The second story, Breaking Quarantine, details the escape of Sergeant Avery Johnson from the clutches of the Flood, immediately after Keyes' squad is overrun during Halo. Due to a pre-existing medical condition, the Flood parasites cannot infect Johnson and attempt to kill him instead. Whereas the Flood are only hinted at being intelligent in Halo: Combat Evolved, the Halo Graphic Novel shows the Flood have a hive mind, assimilating the knowledge of their hosts rapidly. Lee Hammock, writer of 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".
Halo 2 and Ghosts of Onyx 
The Flood make a return appearance in Halo 2, first after being released on a Forerunner facility near Halo, and again on Installation 05 or "Delta Halo". The Flood on Delta Halo are led by the Gravemind, a massive creature that dwells in the bowels of the ring. Gravemind brings together the Master Chief and the Arbiter and tasks them with stopping the Covenant leadership from activating the ring. In the meantime, Gravemind infests the human ship In Amber Clad and crashes it into the Covenant space station of High Charity. Once there, the Flood sweep through the city, before Gravemind appears and questions Cortana. As the Flood spread across Halo and infect High Charity in Halo: Ghosts of Onyx, the Covenant form a blockade in an effort to prevent the parasite from leaving its prison.
Halo 3 
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 the cauterisazion of the only site of outbreak, in the East 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. 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.
Halo 4 
The Flood do not appear in Halo 4's campaign, but they do appear in the Halo 4 multiplayer mode "Flood". Flood is a spiritual successor to Halo 3's and Halo: Reach's Infection mode (Reach also did not include the Flood), where the infected players play as a Flood form with a unique energy sword known as a claw.
Forerunner Saga 
It is revealed in the novel Halo: Silentium that the Flood were once part of the Precursors, an ancient race that was said to accelerate the evolution of a species and "shape galaxies". 10 million years before the war between the Flood and the Forerunners, the Precursors created many races including humanity and the Forerunners. The Precursors denied the Forerunners the Mantle of Responsibility and chose to pass it on to the humans (which would apparently lead to the Precursors "erasing" the Forerunners). In defiance, the Forerunners hunted the Precursors to extinction, however a few of the latter survived and went into hiding; several transformed into powder until it was safe to reappear. But time rendered the powder defective and it brought disease and sickness which the Precursors saw as "good" as they vowed that none of their creations will rise and challenge them again. The Flood was unleashed on the galaxy first starting with a space-faring civilization of prehistoric humans.
Cultural impact 
The Flood have been featured in four series of Halo action figures, produced by Joyride Studios. For Halo: Combat Evolved, Joyride produced a Carrier Form and Infection form bundle. Halo 2's series contains both a human Combat Form and Infection form (bundled with the Master Chief), which were released after the video game. Reviewer Aaron Simmer of Armchair Empire.com expressed the sentiment that Joyride's models could not totally capture the ghoulishness texture and detail of the Flood. The action figures for Halo 3 were produced by McFarlane Toys and although the first series did not feature the Flood, the third series featured a human Combat Form.
Critical reception 
The surprise appearance of the Flood during Halo: Combat Evolved was seen as an important plot twist and a scary moment even after repeat playing of the game. Gamasutra, writing about video game plots, gives the example of the Flood not only as an important reversal to the story of Halo, but a textbook example of how games and their stories are made more interesting by twists in the plot. Mark Binelli of Rolling Stone credited the appearance of the Flood as an excellent way to keep players on their toes by forcing them to adjust their strategies, as well as being a twist as radical "as if, several levels into a game of Pac-Man, the dots suddenly began to attack you".
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. GamesRadar's Charlie Barratt listed the Flood as the worst part of Halo, contrasting what he considered fun, vibrant and open levels before the Flood's appearance with confined spaces and predictable enemies. The Flood have been recognized as some of the greatest game villains. 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". Electronic Gaming Monthly ranked the Flood in their top ten list of "badass undead". MTV considered Flood possession in Halo 3 as a "great gaming moment" of 2007, 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." IGN listed the Flood as the 45th best video game villain, describing them as one of the most hated video game villains in the history of video games.
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