The Flood

The Flood are a parasitic alien life-form that infects and assimilates sentient life. Their Forerunner-era species name is given as Inferi redivivus, meaning roughly “the dead revived”. Their ultimate origin is somewhat mysterious. The most accepted explanation: Precursors transformed into or seeded the Flood as a vengeance or final measure against the Forerunners. Though this is not common knowledge.
Their home-galaxy/origin is extragalactic/unknown. They are not native to the Milky Way, and their biology is alien in structure.

The Flood’s basic mode is to infect a suitable host: intelligent beings with sufficient biomass and neural capacity. They subvert the host’s nervous system and integrate memories and flesh.
They have many “forms” or stages:
- Infection Forms: small, parasite-pods that latch onto living (or dead) hosts and convert them.
- Combat Forms / Carrier Forms / Pure Forms: once a host is converted or biomass is amassed, the Flood can generate specialized battle units.
- Gravemind: a highly advanced Flood intelligence node that arises when the flood biomass and connectivity reach a threshold. It coordinates large-scale behavior.
The Flood prefer close quarters/assault tactics (swarming, melee, infecting) but in cases also use vehicles, weapons etc.

Forerunner-Flood War: One of the most important chapters. The Forerunners found the Flood about ~97,745 BCE and over ~300 years fought them across the galaxy. Eventually the Forerunners activated the Halo Array (superweapons) to wipe out sentient life and thus deprive the Flood of hosts and biomass.
Because of the scale of the threat, the Forerunners even sacrificed their civilization to stop the Flood.
As a faction they differ from the others (UNSC, Covenant, Forerunners) in that their purpose is not political or ideological but biological: assimilation, survival, consumption. They represent an existential threat rather than a typical military power.

The Flood’s strategy is principally: overwhelm. They use numbers and infection rather than finesse.
- They infect as many hosts as possible, convert them, then use the hosts’ biomass and knowledge.
  They can exploit technology and tactics of infected hosts (vehicles, weapons) though they are less accurate or refined when using them.
The Gravemind stage gives them strategic coherence: it allows them to coordinate, manipulate other factions, exploit technology etc.
The threat they pose is one of galaxy-scale extinction: because if the Flood are not contained, they consume host after host, world after world, until they dominate or destroy sentient life.

They are used as a “horror” element: visually horrific, unpredictable, body-horror parasitic menace rather than conventional enemy. This increases the stakes in the story. The Forerunners created the Halo Arrays to starve the Flood of their food sources, sentient life. The first firing of these arrays is what wiped out the Forerunners, aside from extremely rare holdouts that survived through Forerunner Shield-Worlds like Requiem. It is unknown how many are truly left alive, but it is evidently very, very, very few.

The name “The Flood” reflects their nature: a wave of infection and assimilation.
Their Latin designation Inferi redivivus evokes “the dead returned” or “resurrected from hell”.
Although they appear to be mindless zombies, in fact they have a hive-mind layer (via the Gravemind) and are capable of advanced tactical manipulation.

As a faction, the Flood raise the stakes far above “merely winning a war” — they embody potential extinction of life itself.
They serve as a check on all the other factions. Against the Flood, human vs alien politics pale in comparison to “survive or be consumed”.
Their presence enriches the Halo universe by adding cosmic-horror and parasitic-infection themes into a sci-fi military universe.
In gameplay terms they provide a different enemy style (infection, body horror, swarms) versus typical ranged combat.