Mephiston

Calistarius of the Blood Angels drowned in the Black Rage at Hades Hive and should have been another Death Company name on a wall. He clawed back through it, took the name Mephiston, and became Chief Librarian with a psychic weight the rest of the Librarius treats as a weather warning.

He has died and been remade more than once, walked the Devastation of Baal, and still treats the Rage as a door he has learned to shut from the inside. Dante keeps him close. Other Blood Angels keep a little distance, which is sense rather than insult.

Lord of Death is not a courtesy title. It is an account of what it cost to stay a Blood Angel after the Rage had already taken him.