Keritraeus is an Ultramarines Primaris Librarian who served in Lord Commander Guilliman's Indomitus Crusade. In his service to the Crusade, Keritraeus was part of Lieutenant Cassian's Ultramarines strike force, which destroyed the flesh-cults that had overthrown the Imperium's rule on Knossa. Afterwards, they left the world to rejoin the main forces of the Indomitus Crusade, but disaster struck when their Strike Cruiser Primarch's Sword was hit by a Warpstorm. Though the Strike Cruiser eventually escaped from the storm, it was left heavily damaged and the Ultramarines found themselves stranded in the Kalides System. With the Primarch's Sword unable to enter the Warp, Cassian consulted with Keritraeus and Chaplain Dematris on their best course of action. After much consideration, it was decided they would go to the nearby Imperium world Kalides Prime, so they could repair the Primarch's Sword and rejoin the Indomitus Crusade. Cassian also agreed with Chaplain Dematris, that they might face a world in the grips of heresy and the Lieutenant ordered his forces to prepare for battle, as they began to travel to Kalides Prime.