The great devourer.
The Flood is a voracious hive-mind parasite that seizes, converts, and controls sentient hosts to perpetuate itself. Infected individuals have their nervous systems suborned by the parasite, turning their bodies into puppets while their memories and flesh are assimilated and exploited to further the infestation.
Each infected host is linked to a distributed intelligence, allowing for interstellar coordination and the formation of increasingly sophisticated consciousness nodes. If the Flood is not stopped it will eventually manifest a malevolent meta-intelligence known as Gravemind.
Flood are composed of a highly adaptable and resilient neuron-like "super cell" that has no analogous structures with other life in the galaxy. These protean cells are the building blocks of all Flood and can be configured into any shape or pseudo-organ necessary to sustain the biological and organizational growth of the parasite.
Encysted Flood cells form spores which can parasitize both sentient and non-sentient species, rapidly consuming the host and transforming their biomass into an incubator for larger, more mobile Flood combat and infection forms. There is no inoculation against infection, no treatment to slow the parasites' growth, no reversal of its transformation. The only effective countermeasure is containment and incineration.