A species of intelligent space firing I.A parasites created by the legendary Precursors to exterminate species they considered not worth the Mantle of responsibility. The Flood only thinks one thing; Spreading. Once present, the only way to exterminate it is by destroying every single spore of it. Basically, everything.
The Flood is able to consume every organic matter and as well mechanical, able to use them against their "food". They can also wield any kind of weapons. Once with enough mass, a Gravemind will be born and the Flood will become intelligent and smart. This was the ultimate downfall of the Forerunner, with over 3 million world and billions of ships. The Deacon Gravemind know as the Primordial was able to strike the Forerunners from the shadows, turn their military A.i against them and defeat the Empire on a exhaustive war.
Since the Flood can only consume and has no fear and doesn't need resources. Defeating enemies is easy to them, even if takes time. A single flood spore can destroy a species and take over an entire planet.