The Flood Den, also known as Flood Barracks, 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, but while dormant, it will not produce or release any kind of Flood form, and given enough time, the Flood Dens will recover and begin producing more Flood forms. It is speculated that the Flood Dens 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. If this is the case, it probably "stores" Infection Forms.