The Dark Cells, or Black Cells as they are sometimes called, are shadowy oubliettes located deep beneath the Imperial Palace on Terra that contain entities and artefacts that date back to the Age of Strife that could be used to annihilate the Imperium of Man itself if they were to break free or be used by those with nefarious purposes. It is unknown if the Dark Cells are the same location as the Vaults of Rython that were used for an identical purpose by the Legio Custodes before the Horus Heresy.
Guarded by the Shield Host of the Adeptus Custodes known as the Shadowkeepers, the technology used to keep them sealed includes runic locks, psychic wards and sanctic circles. The corridors of the Dark Cells are patrolled at all times by a hundred Custodians of the Shadowkeepers. Although neither light, nor sound can escape from the cells, the dread air surrounding the facility is capable of even putting the iron-willed Custodians on edge.
Even worse, when the Great Rift opened, the Custodes discovered that several of the cells actually now stood empty, the entities and artefacts once contained within spirited away by some unholy force to curse the galaxy once more. Fearing the consequences of such dread remnants of Old Night falling into the wrong hands, the Shadowkeepers at last sent warriors out into the galaxy. These jailers must trammel that which should not be, slaughtering all who seek to impede them, before returning their foul prizes to the cells where they belong.
- Codex: Adeptus Custodes (8th Edition), pg. 23