Flood Barracks

The Flood Barracks (called "Flood Den" in the final version) are organic structures that ceaselessly churn out a variety of Flood forms and are featured in Halo Wars.

They can't be destroyed by "conventional" weaponry, but players can damage them enough to force them into a withered, dormant state. The effect is only temporary, and given enough time, the Barracks will recover and begin producing more Flood forms. It is speculated that the Flood Barracks will only produce Pure Forms as it is a natural "building" and Combat Forms require a host to be created, although it has been stated that the flood have not achieved the "Pure Form level" at this time, as the Gravemind has not been fully spawned yet, and if that's the case, it probably "stores" infection forms, and constructs biomass, since on the picture shown, the flood biomass seems to be built around a Covenant Citidel, likely having plenty of hosts to send into battle.