Corbulo

Corbulo is Sanguinary High Priest of the Blood Angels, Keeper of the Red Grail, the face so close to Sanguinius that recruits have to be told he is not the primarch come back. He reads the Flaw the way other apothecaries read a pulse, and Baal has needed that more than it needs another captain.

The Grail is in his keeping. So is the Chapter's future, one gene-seed at a time. He has stood through the Devastation and still talks about blood as a duty rather than a curse, which is why the priesthood follows him when the Flaw is doing its worst work.

Dante commands the war. Corbulo keeps the brothers able to fight it.