Forerunner Relic
From Halopedia, the Halo wiki
A Relic is a Forerunner structure found in Halo Wars, they hold the keys to the containment doors, and thus can be used to release Flood upon an enemy. They must be garrisoned with troops to release the seal. They can only be found on multiplayer map Release.
Trivia
It is currently unknown why the Forerunners would build a way to ever release the Flood. Though, It is possible they were built before the war against the Flood started in full scale.
