The Flood

is a conglomerate of highly virulent parasitic organisms that can reproduce and grow by consuming sentient life forms of sufficient biomass and cognitive capability. it can infect and mutate Humans and Other species, such, into Combat Forms. They are widely considered to be the greatest threat to the existence of life

The Flood was created over ten million years ago, as a last-ditch Precursor contingency plan to prevent total eradication by their Forerunner counterparts, and also a form of revenge, as well as a coupling to their unification ideology. The Flood was discovered by the Forerunners while exploring the planet Seaward, sparking a 300-year war between the two factions. Initially, the Flood lacked any strategy, but they were outnumbered by billions of Flood forms dedicated to the assault, and every member of the Forerunner population was a potential host. When the Forerunners realized that normal tactics were not enough against the Flood, they created the Halos, a set of last-resort superweapons capable of destroying all sentient life in the galaxy, thus ending the Flood. They finally decided to activate them when the Flood attacked the Forerunner defense line. They also built the Shield Worlds, artificial planets in which living beings could be protected from the effects of the Halos. Finally, the Forerunners decided to activate the rings. The rings were activated from the Ark. The Forerunners ensured that life continued, preserving embryos and living specimens. The Flood were eventually eradicated after the rings were activated. Only a few surviving specimens were contained in high-security research facilities.

The Flood, a virulent species of parasite, is an anomaly to all known biology, and an anomaly to regular life It is extremely adaptive, and mutates hosts through different stages of accelerated "evolution," where the body of biomatter accelerates its own evolution to the point of a massive biological breakdown, whereby the infection can first take place. The species can reproduce without host bodies, although this is described as a last-ditch effort, and only performed by pure Flood forms. would seem that the Flood can infest any living organism, but that they prefer hosts with a large brain capacity to accelerate the formation and expansion of the Gravemind hive intelligence and the Flood intelligence as a whole. They are only as intelligent as the beings they infect; the more cunning the enemy, the more dangerous the Flood will be.

Infection forms selectively target other species that possess sentient intelligence and are of sufficient biomass, and can infest living or lightly wounded dead bodies. Infection forms have long tendrils that pierce the skin of the host and find their way to the spinal cord. They then synchronize with the host, matching frequencies with the host's nervous system and killing them almost instantly From there, they take control of the host body, replacing whatever mind originally resided there with the ravenous and voracious psyche of the Flood, although certain higher-level cognitive abilities, such as combat techniques and technical knowledge which may be useful, are retained, scanned from the victim's brain after death. Despite this lingering of certain memories, no trace of the original mind's personality remains

While baseline neurological assimilation is the same regardless of host species, The Flood's infestation includes physiological mutilation, which is species-specific.

Higher-level species capable of combat, are selectively infected to become Combat Forms. Combat Forms retain the general profile of their original species, although they are significantly mutated. Organ systems are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hitbox-selective incapacitation impossible; organ functions are decentralized, thus, decapitation does not kill a combat form Combat forms also generate tentacles very quickly without regard for the skeletal structure of the host, with tentacles protruding from the combat form at odd locations, typically localized to the chest axial regions, such as the neck, armpits, or torso. A single arm is oftentimes mutated to a claw like or tentacle-like structure that splits into multiple angular, hand-like structures, replacing the hand's usage, though it will still be there. The musculature of the combat form dramatically changes as well, and the form is given prodigious arm and leg strength, allowing for heavy blows with its arm-like structures and for incredible speed and jumping height with its legs. The Flood are also prodigious climbers, able to scamper on walls with little hindrance to the organism

The Flood go through four distinct developmental changes.

Feral: stage is the lowest form: communicating via pheromones, and have the natural instinct to harvest enough calcium to establish a viable Gravemind.

Coordinated: stage is when the Flood becomes truly dangerous; they are now controlled by the Gravemind that was created in the first stage.

Interstellar: stage allows the Flood to take control of most technology and spread throughout the galaxy to infect more hosts.

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.

-Form of infection: It is a small individual that begins the Flood cycle, it is about the size of a soccer ball, it releases spores that paralyze the chosen host, it enters its body particularly in the neck or chest, this individual changes the carrier of the Flood in its DNA and causes mutations such as the rupture of the thorax and neck to unite its nervous system with the host and have control over it to make the species survive. -Combat Form: This Flood is the mutant form of the race, the individual is paralyzed and used by the host, in all cases the Flood plays a little with its new body. The Flood only occupies bodies with a certain intellectual capacity, but if it were so, everyone would have to transform into combat forms, but it is not so, the Flood chooses the race with a specific coefficient and at the same time a body with enough muscle mass to resist the damage that the Flood generates in the body. Commonly many believe that the chest is the weak point but according to observations it is the head since without the host's brain the Flood cannot use the body and is totally useless for infection. -Carrier Form:The Flood Carrier Form is created from a Combat Form that, when no longer useful, begins to produce spores inside itself, swelling with gases. Its function is to transport Infection Forms. They lack any ability to inflict damage by conventional means, they only adopt suicidal behavior by trying to get as close as possible to their target and then explode and release the Infection Forms. They can jump very small heights and fall easily. Only when they explode do they cause damage. -Proto-Gravemind: The Proto-Gravemind, also known as the Brain Form or 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-fledged functional Gravemind. Its creation allows the Flood to move from Feral Stage to Coordinated Stage. -GravemindThe Flood's brain is called a Gravemind, The Gravemind can only be formed if it obtains enough bio-mass to build itself, it controls the entire Flood and can use any form to communicate or act, without a Gravemind the Flood tends to be disorganized and chaotic but just as dangerous, this form of the Flood absorbs all the memories of the infected beings so it always tries to take characters with information or commander capabilities.