As an unexpected addition to the original game, The Flood became Master Chief's true foe in the Halo series. The story revealed that this parasitic creature probably came from outside the Milky Way galaxy, and was what caused The Forerunners to construct the Halo arrays in the first place. It was decided that destroying all sentient life in the galaxy (thus denying The Flood any source of food or sentient life to infect) was more practical than the conventional methods of control that were already failing. Many species were also preserved and kept safe by the Forerunners so that they could later evolve and restore life to the galaxy, although the Forerunners still decided to sacrifice themselves.
In the bestiary booklet that came with Halo 3, it is revealed that the Flood utilize a special type of cell (Flood Super Cell) which can be described as a 'thinking muscle', and can be rearranged to resemble any necessary organs.
The Flood has many forms, but begins as a small parasite born from an egg sack. Its purpose is to infect any sentient life it comes into contact with. Once infected the host can be evolved and twisted into fighting units, some of which can use weapons, or swollen up to become essentially another egg sack to breed new parasites. The main Flood forms are;
The most basic unit of the flood, this small creature is known as a 'spore'. They often attack in huge quantities and can do fatal damage to Master Chief if his shields are down. From Halo 2 onwards, they can reanimate killed Flood corpses, and in Halo 3, they can reanimate any human, Elite, or Brute corpse instantly, they can also reanimate flood corpses unless the legs, arms and body are melee'd/shot at enough.
These are the foot soldiers of the Flood. They are formed out of Elites, humans, or Brutes, and are highly versatile, capable of using a variety of weaponry in addition to their deadly melee attack, using equipment such as shielding and cloaking, and even piloting vehicles.
These are huge, bloated, slow-moving Flood that try to kamikaze the player, exploding when shot or when close to the player, and releasing an explosion that does significant damage and a wave of Infection forms. A carrier can easily be destroyed, along with ALL of its spores, with a simple Plasma grenade (or incendiary, but they are extremely hard to find). Not only does it kill the carrier, but it can be done at a distance and it often kills all of the spores that the carrier releases.
Only featured in Halo 3, this creature is capable of shifting between three forms;
- Stalker form: These Insect-like creatures are very agile and can jump huge distances, they can also crawl on walls. They usually do not attack the player, unless the player is standing in their way; however, they can do serious damage to unshielded players.
- Ranged form: Usually after the stalker form sees the player, it then crawls to a high ledge, or even a ceiling, and will fire relentlessly in the form of sharp needles. These forms are inclined to stay in groups, firing all at once, or at different times as to pin the player into a single area of cover.
- Juggernaut form: Rivaling the Covenant Hunter in it's size, this form is undeniably the strongest form of The Flood. Once the Pure Form turns into this living tank, it usually can't/won't turn back. The Juggernaut can take a tremendous amount of damage, about 4-5 Energy Sword swings or 3-4 rockets. They are best taken out at range, since their melee attack can remove shields with two hits (normal), or one hit (Heroic and Legendary).
The Flood are generally annoying to combat; after large fights depleted shields are likely. Additionally, these enemies will often sneak-up on the player -- this can result in quick kills. A few tips include: Spraying and Praying, this tactic can get rid of a lot of spores and/or some combat Flood; and using a flame-thrower, as The Flood take massive amounts of damage from this particular weapon.
In Halo 2, there was a type of Flood removed from the game due to the fact it was very glitchy, too hard to fight and spawned in squads of five or more.
This type was called a Juggernaut, although it was very different to Halo 3's Pure Form Juggernaut.
This was twice as tall as the Master Chief and used a number of tentacles to stand. PC players of the game found that Bungie hadn't really removed all of the Juggernaut's coding. When spawned on a PC, however, it will not attack the player nor will it take any damage at all.
The Gravemind is the intelligence that leads the Flood. The Flood go through three stages, Feral, in which they are disorganized and communicate via means of pheromones, the Coordinated state, in which they grow dominant enough to form a Gravemind, and the Interstellar state, which has to do with the acquisition of technology rather than growth or evolution. A Gravemind is a fusion of pure Flood Super Cells (the building blocks of the Flood), and the neurosystems of one or more intelligent, sentient beings. The Gravemind does not 'lead' the Flood in the traditional sense of the word. The influence it exerts has been likened to a human's control over his limbs. In the Halo series, the Gravemind appears to possess great intelligence, twice tricking the Master Chief and The Arbiter into serving him only to betray them.