The Flood is a parasitic virulent life form, known for being a highly virulent and deadly race, which can quickly incapacitate and infect different life forms, but the infected subject must have sufficient body mass to sustain infection, and even animals can be infected.
Origin and History
The flood comes from the Precursors, the alien race that seeded the Halo universe. After angering the Forerunners by announcing that the Mantle of Responsibility was to be handed down to ancient humans, Forerunners commited a genocide against them, to which the Precursors did not defend themselves. Some that escaped turned into a dust, that eventually became corrupted. Upon awakening from the sleep, and seeing what had become of them, they vowed revenge on the creatures who ruined them, and wanted to unite the universe. One of the things that is known is that they thrive in an environment that is Noxious to most other living creatures, as 343 Guilty Spark and Cortana both comment on the Flood’s terraforming abilities, implying their natural environment differs significantly from Earth-normal.
Little is currently known about the Flood. It is known prehistoric Humanity encountered the parasite before the Forerunners, thousands of years before the events depicted in the Halo series. The first known "arrival" of the Flood occurred some time prior to 110,000 BCE, aboard automated starships of unknown origin that came from the Magellanic Clouds. The crashed remains of these ships were first discovered by ancient humans on worlds near the border of intergalactic space. The vessels, which were found crashed on both inhabited and deserted planets, contained millions of glass cylinders filled with a fine desiccated powder, which was found to consist of inert, short-chain organic molecules of unknown purpose. Unknown to the Humans, these were devolved and mutated Precursor bodies.
Humanity studied the material and deemed it to be harmless, but noted that it had psychotropic effects on lower animals. Small dosages were administered to Pheru, popular domestic animals among humans and San 'Shyuum, which resulted in more docile behavior. Unbeknownst to the humans or San'Shyuum at the time, the powder was altering the genetic code of the Pheru, manifesting as improved behavior while slowly changing other aspects of their genome.
After several centuries, more disturbing changes appeared. Pheru treated with the powder began exhibiting furry growths and fleshy protrusions, which other Pheru were compelled to consume. Shortly afterward, the Pheru began suffering widespread abortions and genetic deformities, and many were euthanized or released into the wild. Soon afterward, the humans and San'Shyuum who had been in the presence of the altered Pheru began to exhibit some of the symptoms of the powder-induced "disease."
Affected on a psychological level by the strange illness, infected victims began to consume the sick Pheru, and parts that were discarded also became vectors for the disease. The afflicted then moved on to cannibalism and sacrifice of the uninfected. The infected humans and San'Shyuum proceeded to spread the disease into other systems, which furthered the scope of the Flood outbreak.
At this point, the Flood infestation, which had previously been predominantly a mind and behavior-altering affliction with only relatively minor physical changes, began to reshape its victims into a variety of forms designed to gather victims, engage in combat, and to rapidly consume useful biomass and other resources. The growing outbreak quickly led to a conflict between humanity and the Flood.
During the Human-Flood war, Humanity managed to create a cure for the Flood. This landed such a devastating blow to the parasite that they were not heard from again until the Forerunners' first contact at at G617 g1. Unfortunately, the cure was lost when Humanity was devolved by the forerunners as punishment for the Human-Forerunner war.
Many confrontations and disasters followed, eventually resulting in the Forerunners instituting a variety of construction projects in order to curb the threat. This varied from war fleets to numerous research installations and weapons platforms constructed all over the galaxy. Varying from shield world installations such as the miniature Dyson sphere known as Onyx, to the massive orbital installations known as Halos. The Halos in particular served the dual purpose of quarantined research facilities and more importantly, as the delivery system of a super weapon energy pulse capable of wiping out all sentient life of sufficient bio-mass to sustain the Flood in the entire galaxy.
In the event of a Flood escape despite the containment procedures, and assuming all other strategic possibilities were exhausted, the Halo System was designed to be fired, cutting off vectors by which the flood might spread. 343 Guilty Spark states that the Halo System was fired once before when it became clear that there was no hope of ending the Flood/Forerunner war by conventional means. The plan worked, and although unthinkable amounts of sentient life were wiped out, the Flood was halted from spreading and starved to death for lack of sustenance... eventually allowing the galaxy to re-populate in an un-infested environment. Despite this measure, some Flood specimens survived in the research facilities aboard the Halo installations themselves, entering into a state of suspended animation and surviving for millennia in dormancy.
During the events of the Halo trilogy, the Flood were re-awakened when both Humans and Covenant began to uncover and interfere with the now ancient and abandoned Forerunner Installations. Unknown to the rest of their respective species, the Flood were first encountered by the Human ship 'Spirit of Fire' and an unknown covenant ship commanded by The Prophet of Regret and the Arbiter when both ships stumbled upon and fought over an unknown miniature Dyson Sphere disguised as a planet.
Several confrontations took place involving all three factions, and the reason the Flood was active and uncontained at this particular facility are unknown; though it is likely that it was a battlefield during the Flood/Forerunner War, the Flood being the obvious victors. It is known the humans were unable to pass along information about this threat back to the main UNSC government or forces due to extensive damage to their ship. Whether the Covenant were able to escape and communicate the threat is not explicitly stated, but it is heavily implied as they seem to conduct themselves with a degree of foreknowledge aboard Alpha Halo 20 years later.
During the events of the first Halo, the Flood were accidentally unleashed by the Covenant, decimating all other factions on the Installation and spreading at a frightening rate. The Covenant retreated, suppressed and sealed the entrances but a team of UNSC marines lead by Captain Keyes of The Pillar of Autumn, unwittingly unsealed the Flood, sealing the fate of Installation 04. With the Covenant and UNSC forces battling one another, the Flood saw an opportunity to spread further than the boundaries of the Installation and set their sights on spaceships currently in Orbit. There were three recorded attempts to board and repair ships and leave Installation 04.
- The first was when a force of Flood managed to take over the Covenant supply/agriculture ship Infinite Succor. A strike team led by Rtas 'Vadumee (Half-Jaw) was sent to investigate a distress signal from the ship. After a series of loses on the Rtas 'Vadumee side, the only way to win the battle was to destroy the ship which was done by opening a slip-space window to the nearest sun, the Prophet on board tried to change Rtas 'Vadumee's mind to no avail. It is presumed the ship, with the Flood on it was destroyed entering the sun, but it is not confirmed.
- The second attempt was when the flood tried to board and leave on the Truth and Reconciliation. This attempt was less successful, as Covenant forces were able to defend their ship, but due to their dwindling numbers they were instead over run by Marines commanded by Major Silva. Major Silva was about to leave Installation 04 on the Truth and Reconciliation but unknown to him or his team-mates they had stowaways. Jenkins insanity warnings, due to his partially flood transformed body, eventually got through to First Lieutenant Melissa, an ODST under Silva's command, and she destroyed an important energy transfer cable with a grenade, this disconnected the connection between the engines and controls. The ship fell back into the gravity of the Installation and everyone aboard was killed.
- The third attempt was to use a badly-damaged Pillar of Autumn. It was heavily damaged due to wounds inflicted in a fire fight with Covenant ships and the later crash landing: this meant the ship was over looked by the Covenant. When the Flood infected a number of Marines and workmen who served on the ship, they were able to acquire the knowledge about the ship systems and began repairs. They were interrupted by Covenant forces, the Master Chief and 343 Guilty Spark with Sentinels. The Master Chief later set the fusion reactor core on the Pillar of Autumn to overload which caused the resulting explosion to destroy Installation 04.
The release of the Flood on Installation 04 prompted the Monitor of the Installation, 343 Guilty spark, to seek a reclaimer to retrieve the Index. Conveniently, there was one on board the installation: Spartan-117, John, Master Chief Petty Officer.
After the Master Chief retrieves and begins to use the Index, the true nature of the Halo rings is revealed by the Chief's own AI, Cortana. Discovering that Halo's defense system is a weapon designed to kill all sentient life in the galaxy, which the Flood need to spread. When confronted with this information, 343 Guilty Spark states that the installation technically only has a maximum radius of twenty-five thousand light-years, but that its pulse would trigger other similar installations as well, killing all sentient life in the galaxy.
Heretics had commandeered a Forerunner station in the gas giant Threshold that Installation 04 orbited. The station was already holding The Flood for defensive study by the Forerunners. The Flood overran the station, and the Arbiter cut three cables holding the station above the Gas Gaint, plunging it into the heart of the planet.
The second outbreak happened on Installation 05, also known as Delta Halo to the humans. This outbreak by the Flood was far more advanced and organised, with the Flood managing to take over the Covenant High Charity, using a number of UNSC Pelican drop ships from a crashed In Amber Clad to board the giant floating city. On Delta Halo, Gravemind revealed himself to Master Chief, the Arbiter and Cortana, his reasons for saving their lives were not found until Master Chief saw Pelican drop ships carrying The Flood. Master Chief and the Arbiter were only a diversion for Gravemind's plan. Eventually, Gravemind rooted himself in High Charity most likely using the teleportation grid, and the Flood took control of High Charity. During these events, one of the human AIs, Cortana, was captured by the Flood and interrogated by the Gravemind, who was hoping to corrupt the AI into leaking important information. To stop this from happening, Spartan 117 was dispatched to retrieve or destroy Cortana. During the crusade Master Chief was transmitted messages from Medicant Bias, a corrupt Forerunner AI. When he finally confronted the Gravemind, it was revealed that the Gravemind was going to rebuild his invasion force on Installation 04 and Installation 00, the Ark, by crashing High Charity into Installation 00.
Biology and Life Cycle
The Flood appears in four primary forms: Infection, Combat, Carrier and Pure forms, but also have more mysterious forms such as the Gravemind form. The flood will consume all life it comes in contact with, and can mutate its hosts into various forms to help it do so.
There are three stages in the evolution of a flood colony:
- Feral stage is the lowest form: communicating via pheromones, and have the natural instinct to harvest enough calcium to establish a viable Gravemind. (Seen in Halo: Combat Evolved and Halo Wars)
- Coordinated stage is when the Flood becomes truly dangerous; they are now controlled by the Gravemind that was created in the first stage. (Seen in Halo 2)
- Interstellar stage allows the Flood to take control of most technology and spread throughout the galaxy to infect more hosts. (Seen in Halo 3, this is the stage that wiped out the Forerunners)
- Intergalactic is the last known stage the Flood enters, theoretically, in which it utilizes all captured technology to depart to uninfected galaxies to further replicate. (End of Forerunner Saga)
The Flood Infection Form is the first stage of the virulent Flood xenoform. It is the most commonly seen stage of the Flood, and is the form responsible for spreading the Flood infection.
The Flood Infection Form might appear at first glance to be little more than a harmless and repulsive creature, but once it gains physical contact with a biological host, it instantly becomes a deadly foe. Any foe without shielding protection will be prone to instant death or infection, although the potential host can fight back with limited success when the Infection Form attempts to infect them. Small tentacles underneath its body penetrate the skin, tap into the victim's spinal cord, and unleash an attack on the host's nervous system via direct contact with the spine, killing the host almost instantly, though in rare cases leaving the victim conscious. Once this is accomplished, it rewrites the neural pathways of the victim's brain with its tendrils, forcing a resonant frequency match between its neural signals and the host's signals. Then, it releases the Flood Super Cell which overwrites the host DNA and causes rapid mutation.
After this is accomplished, it starts to morphologically augment the body, liquefying organs and redistributing biomass to create tentacles on the left arm from the calcium in osseous tissue. The central mass of the Infection Form will finally nest itself within the chest cavity of its host. This mutagenic process can occur within a matter of seconds. In exceedingly rare cases, such as if the Infection Form has been damaged or is incredibly old, it will go through the mutation process without killing the host, leaving them at least partially aware of what is going on, but unable to move or act. This was the fate of UNSC Private Wallace Jenkins.
If the infection of a particular being is not possible, that organism will be totally disregarded by Infection forms, deferring to other Flood forms the task of slaughtering it. It is likely that the Flood will use the biomass of the incompatible body for sustenance later. An Infection Form may also abandon its host if the corpse has been heavily damaged to look for a new one. Several Infection Forms, sometimes ranging from 5 to 15 at once, can attack certain victims. Infection forms appear at first glance to be quite small, but upon comparing them with the average Human, they are roughly the size of a man's torso.
Infection Forms also reanimated disabled Combat Forms if there are any nearby, following the same process as with uninfected hosts without releasing the Super Cell.
Energy shielding negates the effect of the infection form by causing it to explode on contact in a small, damaging cloud of gas. However, if no shield system is present on the target, the infection process begins.
At this time, the only known human to be incompatible with Flood infection forms is Staff Sergeant Avery Johnson due to his supposed contracting of Boren's Syndrome from the residual radiation of a crate full of plasma grenades on Paris IV. However, Johnson actually contracted this immunity from the augmentations he received as part of the ORION Project. It is possible that this causes all members of the ORION Project to be immune. According to Cortana's analysis of various Flood forms, Kig-Yar and Unggoy generally lack the necessary calcium stores and biomass to be converted into Combat Forms and are instead used as biomass stores and as carrier forms, although Kig-yar and Unggoy Combat Forms have been observed during one engagement. This was most likely to supplement the Flood's combat capabilities as they may have been unable to muster enough proper Combat Forms to fight the enemy.
The Mgalekgolo's lack of a central nervous system and nature as an invertebrate colony consisting of multiple worms make them immune to Flood infection. The invertebrate nature of the Yanme'e, along with their hard, chitinous exoskeleton, would seem to make them immune as well.
An Infection Form is capable of losing its infectious potential over time. This was experienced by Private First Class Wallace Jenkins during the raid on the "Covenant Weapons Cache" on Installation 04. This aged version of the Infection Form must have been kept in captivity for far too long, thereby reducing its potential to completely transform its host into a Flood Combat Form. Jenkins was only partially controlled, as there were periods of time where he could control his body and mind to some degree. For example, he was able to tell where the Flood were hiding, and saved a Marine Battalion.
A Flood Combat Form is a sentient being that has been infected by the Flood parasite via an Infection Form or Flood Spores, mutating the victim into a basic combat unit for use by the Flood. They are extremely dangerous, but can be easily defeated when alone or feral. However, when under the control of a Gravemind they can be coordinated and deadly.
A Combat Form is not a natural organism, but rather a mutated host infected by parasitic Flood Super Cells implanted by an Infection Form. Successful creation of a Combat Form requires a host with sufficient biomass and calcium storage, and an Infection Form to infect the host. After mutation begins, the infection form attaches with one of its tentacles, which it uses to establish a neural connection that allows it to command the host's nervous system, thus rendering command of the legs and arms useless. During the infection process, the host's internal organs are liquefied, and the nutrients from them are used to develop the tentacles and other appendages.
A host becomes a Combat Form after an Infection Form enters its body. Transformation involves brutal mutations that makes the host more suitable for combat. This process is presumably very painful and terrifying, and will kill the host; though, in certain circumstances, the host is still alive. Once fully transformed, the Infection Form has total control over the host's body and changes the physical appearance of the host to better suit its own needs (although an infected host will be roughly the same size as it was before). On rare occasions, this effect may be weakened by time in stasis for the Infection Form, allowing the host to regain some control, as in the case of Private First Class Wallace Jenkins. Combat Forms retain the host's previous attributes, therefore they can wield weapons, and drive and board enemy vehicles. However, Infection Forms do not require their hosts to still be alive for infection. Even dead hosts are suitable for conversion into a Combat Form; infection forms can infect recently deceased hosts and reanimate them in much the same way as they would living hosts. Although the brain tissue remains dead, the victim's biomass and calcium reserves are sufficient to warrant infection.
In the initial stage of a Flood outbreak, the Feral Stage, Combat Forms communicate using pheromones and have one instinctual behavior; to secure new hosts to strengthen the local controlling intelligence. Once a sufficient store of biomass is available the Flood enters the Coordinated Stage, and begins forming Pure FormFlood, with the Combat Forms being used either as defensive units or calcium/biomass reserves.
Infected hosts are rapidly digested while being converted, which leaves the host severely ravaged and decayed, much like a corpse that has been exhumed a few weeks after its burial. Flood infection is a rapid and efficient biological process, as it can take under a minute in certain cases. A group of Flood will occasionally go after Sentinels if they are perceived to be the greatest threat in the area, although they are unable to be infected by the Flood due to their robotic and inorganic nature.
Carrier forms are formed from older Combat Forms or from bodies that are unsuitable for combat. Lieutenant McKay theorized that Unggoy and Kig-Yar couldn't become Combat Forms and instead directly converted into Carrier Forms, though this theory has been disproved.
Combat Forms can be converted when they reach the end of their life span, or become too damaged to fight. Carrier Forms perpetuate the Flood species by acting as incubators for newly created Infection Forms and keeping them safe until they're fully developed. Eventually, when a Combat Form has lived long enough, it will evolve into a Carrier, regardless of its status.
Once the Infection Forms are ready, the Carrier simply explodes spontaneously, sending the new Infection Forms flying into the air. As soon as the Infection Forms are released, they begin searching for new hosts. It should be noted that Carrier Forms won't wait until full maturation has occurred.
If potential enemy hosts are nearby, it will simply get close to them and explode, sending the Infection Forms flying at the unsuspecting and often injuring or killing enemies. The explosion of a Carrier Form is similar to the explosion of a fragmentation grenade, and deals damage to anything nearby.
The Carrier Form has a simple mindset, which is to release the Infection Forms it carries to a suitable host. Other than organic bodily sounds, they cannot speak or make any other sound. Though they can somehow communicate with other Carriers, possibly through a Gravemind or Brain Form, to inform them of a potential sentient being.
The Proto-Gravemind, also known as the Proto-Compound Intelligence, is an advanced Flood form created from the biomass of the most intelligent living organisms available, with the ultimate goal of creating a fully-functional Gravemind.
Proto-Graveminds are a coagulation of multiple other Flood forms all merged. It appears that when an advanced task is at hand for the Flood, such as piloting a starship, they will create a Proto-Gravemind to do that task by merging as many hosts as possible with that knowledge, such as former pilots and officers. Rather than completely destroying a victim's consciousness, as a normal Flood form generally does, it interrogates its victims slowly, allowing their consciousnesses to feed it information. The form tortures the host's mind with a loud buzzing sound, thus erasing all thought. Only after the host is stripped of all knowledge is the host killed by the Proto-Gravemind, although it is sometimes possible for a host to die before the Flood form has a chance to assimilate all of its memories.
A notable feature in the creation of a Proto-Gravemind is that San'Shyuum are often used to form the core "mind" of the Brain Form, such as Regret, or the Minister of Etiology. The biggest difference between the infected San'Shyuum and that of the conventional Combat Form, is that examples seen thus far have maintained their personalities to an extent. Other infected forms are subjected completely, to the control of the Infection Form within them and generally display none of the former personality traits of their hosts. On the other hand, infected Prophets have demonstrated the ability to act the same way that they did before they were assimilated. As the Prophet species are considerably weaker and more frail than many other Covenant species, the Prophet would not be a suitable creature to create a Combat Form, and their intelligent nature likely make them more suitable candidates as the main host of a Proto-Gravemind.
Proto-Graveminds are only seen piloting more complex vehicles that require trained pilots. While Combat Forms can easily, albeit clumsily, pilot smaller vehicles like Warthogs, Scorpions, Wraiths, and Ghosts (at least once the Flood infection has reached the Coordinated Stage), Proto-Graveminds are needed for larger vehicles such as a spaceship. Since the Proto-Gravemind does not appear to move at all, it is likely that it directs nearby Combat Forms to pilot the ships, similar to the way a fully-formed Gravemind will direct Flood forms to perform complex tasks or use the Flood Super Cell to pilot the ship.
Unlike all other known Flood forms, it is apparently immobile and passive in combat. They have never been observed to move on their own, even when other Combat Forms are fighting nearby. In fact, even when John-117 physically punched through the side of the Proto-Gravemind present aboard the Truth and Reconciliation during the Battle of Installation 04, the Flood form did not react at all; it is likely that locomotion is only gained after the transformation into a true Gravemind is complete. Proto-Graveminds have also never been observed to keep any of their hosts' weapons. They use some of their hosts' legs as a means to support themselves, and one can slightly see some of the recent hosts' bodies embedded in the Proto-Gravemind's mass, albeit deformed.
The Proto-Gravemind must grow incredibly in size to reach its next stage of development, known as a Gravemind, a near-omniscient Flood form made of thousands of host bodies infused with the knowledge of each host that was used to make the Gravemind. Graveminds are created once a Proto-Gravemind reaches a certain "critical mass" and develops a centralized intelligence. This happens only on extremely rare occasions, usually because the Flood is seldom loose on a large enough scale to allow Proto-Graveminds to grow large enough to become a Gravemind. Only two Graveminds are known to have ever been created; however, they seem to have the same memories, since the Gravemind present during the Human-Covenant war possessed detailed knowledge concerning the Forerunner-Flood war and even described the actions of the previous Gravemind during that time period as if they were his own. In essence, because it shares all the knowledge of the previous Gravemind and considers them both to be the same being, the Gravemind of the games is the Gravemind that fought the Forerunners, simply been reborn from another Proto-Gravemind.
The Gravemind (Inferi sententia, meaning "Thinking Dead"), is the penultimate stage in the life cycle of the Flood and is the ultimate intelligence of the Flood. It is a near-omniscient creature with complete control over all Flood forms. A particular Gravemind is the secondary antagonist in Halo 2, and a main antagonist in Halo 3.
A Gravemind starts out as a Proto-Gravemind—a Flood form created by merging the bodies and biomass of numerous sentient life forms; some Flood forms may also be repurposed and merged into the Proto-Gravemind. After a Proto-Gravemind has been created, nearby Combat Forms will continue to supply it with fresh bodies, allowing it to accumulate mass, increase in size, and gain more memories and intelligence from consumed hosts. Eventually, the Proto-Gravemind reaches a certain critical mass and becomes self-aware—a Gravemind. This critical mass tends to be made up of thousands of bodies if using human-sized bodies as a measuring unit.
The Gravemind, and its subordinate Flood infection, will then continue to accumulate. Eventually, when the Flood infection has become too large for even the Gravemind to control—or when no life remains in the galaxy—the Flood infection will reach the Intergalactic Stage, and Flood will leave the galaxy with the intent to create a new Gravemind elsewhere.
Gravemind makes up a distributed intelligence, with a single primary consciousness made up of multiple, smaller instances that seem capable of acting independently. This is shown in Silentium, where the Precursor Gravemind compares itself with Catalog, a group of modified Forerunners networked together into single mind acting in concert with many others.
Due to the brevity of its mysterious appearances in Halo 2, little is known about the Gravemind's physical form. It is a massive Flood form whose "mouth" resembles a fleshy, multi-layered Venus Flytrap. The organ is used to speak and to exhale Flood Spores. The remainder of the Gravemind's body consists of a mass of enormous tentacles. During its time beneath Installation 05's Library, some of its tentacles grew to be miles long and stretched throughout the Installation.
The Gravemind, like the Proto-Gravemind from which it formed, is made of countless bodies and corpses; The Art of Halo states that Gravemind is "literally built from the bodies of its enemies and its own fallen warriors reassembled into a massive, tentacled, and intelligent entity." Gravemind references this fact often, often calling his current whereabouts an empty grave or burial mound—where there should be many graves, there is just the Gravemind. Even his name, "Gravemind," references this: he is the mind of the grave. The consciousness of the Gravemind is formed from the accumulated intelligence and memories of every Flood host ever consumed.
Gravemind is the controlling intelligence behind the parasitic Flood species. In this way, he appears similar to a puppeteer. When a creature is assimilated into the Flood, its knowledge is transferred directly to the Gravemind, and the remainder of its mind is destroyed. In addition, it appears each Gravemind also retains the memories and knowledge of previous Graveminds. This has made the Gravemind virtually omniscient, and ensures that should the Gravemind be destroyed, its consciousness will never truly die as long as some flood forms are left. In an archived conversation with the Forerunner AI known as 032 Mendicant Bias, he compared himself to the AI, describing it as "a single intelligence inhabiting multiple instances" and calling himself "a compound [intelligence] consisting of a thousand billion coordinated minds inhabiting as many bodies as circumstances require." While it is not known how the Gravemind is able to communicate with subordinate Flood forms across the galaxy, his self-comparison to a computer network implies that similar techniques may be used, with each Flood form possibly acting as a networking node and redistributing the Gravemind's commands to other forms. It is widely believed that the Gravemind uses some form of telepathy to command its disparate Flood components.
Gravemind is able to subvert AIs. In the Forerunner-Flood war it was capable of altering every Forerunner AI's basic logical processes, causing it to aid the Flood cause. This 'Logic-Virus' was considered by the Forerunners to be the informational form of the Flood infection, to which Mendicant Bias was the first victim in its 43-year discourse with the Precursor Gravemind.
During the Flood-Forerunner War, when the Gravemind was a galaxy spanning network, it began assembling entire planetary ecosystems into enormous, highly complex structures referred to as 'Key Minds'. Each Key Mind was said to have easily matched a single Forerunner Metarch-level AI (By far the most powerful and intelligent grade of AI the Forerunners had developed and of which Mendicant Bias was an example) for strategic planning and raw computational power.
Due to the Gravemind's understanding and absorption of Precursor knowledge, it was capable of controlling and manipulating Precursor technology in the Forerunner-Flood conflict, which allowed it more effective slipspace technology and the ability to prevent Forerunners from using Slipspace. Now that all Precursor technology has been destroyed, it is unknown if the current iteration of Gravemind was capable of the same thing, though if all flood contain all memories of the prior versions of Gravemind or they are otherwise stores somewhere, it could be surmised that it still retains this ability.
A Key Mind is an exceptionally advanced form of the Flood. Several were formulated during the Forerunner-Flood war to combat the Forerunners with pure strategic might. The use of Precursor technology gave each Key Mind incredible processing power and strategic skill that easily matched that of a Metarch-level Ancilla, such as Mendicant Bias or Offensive Bias. Being a compound, distributed intelligence, each Key Mind was an extremely large and well-developed instance of the Gravemind entity. These enormous collective consciousness composed the leadership of the Flood, coordinating their efforts throughout their war with the Forerunners. Each new instance added to the overall Gravemind's intelligence and computational power.
Pure Forms are highly advanced Flood forms only seen very recently since their discovery. According to research, they are formed when sufficient biomass has been accumulated by the Flood. Calcium from the bones of the Flood's victims makes up the structure of the Pure Form. Through some means as of yet unknown, the Pure Forms have the ability to alter their structure, and possibly the make-up of their structure, as a response to different combat stimulus.
Upon death Pure Forms do not fall apart, but into a pile of limbs and "flood crust"
The Flood Stalker Form is a crustacean-like Flood Pure Form that specializes in stealth and agility. It is apparently made from the re-casted bones of dead enemies. It will frequently mutate into either a Ranged Form or a Tank Form when the need arises. This form sits low beneath your field of view and their attacks can go widely unnoticed until you look down. They are able to crawl on walls and ceilings with ease, and are capable of making enormous jumps much like combat forms. A possible flaw in its AI is that it has been observed to rarely attack players, and normally prefers to mutate itself into one of the two other Pure Forms. It appears to attack more often on the levels Cortana and Halo. Even if they do attack, they are not as strong as the Ranged or Tank forms, making it the easiest of the three to dispatch.
The Flood Ranged Form is a type of Flood Pure Form from Halo 3. It is formed when a Flood Stalker Form is damaged or chooses to mount itself in a stationary position either on the walls, ceiling, or, with less frequency, on the ground. It has a mouth-like protrusion that opens up to reveal a living missile platform of regenerating spikes. These spikes will relentlessly pelt the player if exposed, and are very irritating and oppressive. They also exhibit a behavior unlike any other in the Flood arsenal. When fired upon these forms will curl up to shield their heads, their weak point. To fully disable them the player must stop firing until they reveal their weak point once more, or continue firing until they die, regardless of their defensive posture. When the Pure Form is killed on a wall or ceiling, they will fall off. They can be compared directly with Carbine-carrying Sniper Jackals earlier in the game in regards to range, damage, accuracy and relative annoyance and it is not unreasonable to mistake them for infected drones due to their insectile nature.
Tank Forms are snail-paced in comparison to the other Combat Forms but they make up for it with relative resilience and their devastating melee attack, as they are unable to wield weapons with their arms that are shaped into hardened spikes made from bone (as opposed to whip-like tentacles exhibited by other Combat Forms). However slow, they can run on all fours for a short period, allowing a short burst of speed to overtake a target much like Jiralhanae from Halo 2 and Halo 3. On occasion Tank Forms eject Infection Forms from their 'mouth' as if regurgitating them (not explosively like the Flood Carrier Forms) as a form of short ranged attack, or an attack to counteract Bubble Shields. Like all of the Pure Forms, The Tank forms cannot be dismembered, only disabled.
The Tank Form is similar in appearance to the Drinol, and may have been inspired conceptually by the deleted Covenant race. There is also a notable similarity between the roles of the Tank Form and of the Covenant Mgalekgolo, and is often mistaken as an infected version of a Hunter, however that is impossible considering that Hunters are a colony of worms and the Tank Form is one single mass.
Unlike other forms, Tank Forms let out a roar whenever they spot an enemy. This allows a player to determine if one is nearby and can take appropriate actions to defend themselves. Tank Forms mainly attack with their claw arm, which does damage similar to a Beam Rifle; a swing on Heroic drains the player's shields while a swing on Legendary will kill them. Should they take too much damage, they will use their claw arm to block incoming fire, where grenades should be thrown instead as they don't tend to avoid grenades. In addition, Tank Forms can spew out Infection Forms from their mouths, although this is quite uncommon. If the target(s) are too far from the Tank Form, they will turn into the Stalker/Ranged Forms.
The termonology "flood" is derived from the Latin: Tecktius Flood. This is translated exactly as: "The apocalypse in physical". This is the first of the theories of where the name "flood" is actually derived from. It could also just be a nick