A Librarium, or Librarius is the command and communications center of a Space Marine Chapter's fortress-monastery, and the repository for centuries of wisdom and history, culled from the reports, treatises and memoirs of the chapter's greatest warriors and finest minds.
Librarians hold a functionary rank, describing their role as well as their position. The four battlefield ranks of Librarian, in ascending order of importance, are Lexicanium, Codicier, Epistolary and Chief Librarian. Each of these enters battle ready to support his brothers with a range of psychic attacks and defences.
Within certain chapter fortresses, the Librarium serves as the control center for the base's defence systems.