Mephiston, originally named Calistarius, and also called the "Lord of Death," is the Master of the Librarius and the Chief Librarian of the Blood Angels Chapter.

He is the only member of the Blood Angels or their successors among the Sanguinary Brotherhood known to have defeated the pull of the Black Rage not once, but twice in his life, regaining his sanity both times. He is a Librarian of potent ability, believed to be one of the mightiest psykers in the entire Imperium of Man.