Sacrifices: In every Black Ship's cargo of psykers there are always a percentage who are unworthy: too weak to become psykers with the Guard or Marine Librarians, too vulnerable to serve as astropaths. Instead, their psychic energies are used to feed and sustain the Emperor's body.
Left unrestrained and untrained they would be both doomed and doom - for they could provide a conduit for a daemon to enter the material world. The sacrifices are identified during the long voyage of the Black Ship and transported to the Golden Throne, where their psychic energies, and thus their lifeforce, are consumed by the Emperor to sustain his exsitence.
Inquisitor Jaq Draco formed a brief relationship with one of the sacrifices to be during his own journey to Terra aboard a Black Ship; a young psychic healer named Olva. During his meeting with the Emperor, Jaq Draco discovered that the Emperor subsumed the minds of those he consumed into his own mind, creating a multi-mind which could perform the manifold duties of the Emperor simultaneously.
Source: Draco by Ian Watson, White Dwarf: Issue 140 by various
Submitter: Simguinus
Hive World Terra