Crull was the borderline psychotic Chaos Lord of the World Eaters Traitor Legion who led the Blood Legion of Khorne warband to the Ice World of Lorn V in order to obtain some unknown artefact for the glory of the Blood God.
The Khornate warband and a large Ork horde present on the world had been fighting for a number of standard years until Crull and the Ork Warboss Gorgutz struck an uneasy alliance in order to destroy the encroaching Forces of Order, represented by the Imperial Guard and the Eldar.
This cooperation immediately broke down once Gorgutz announced that he would destroy the remains of the Imperator-class Titan that lay quiescent on Lorn V, which Crull wanted to claim for his own personal gain and the glory of Khorne. Crull was eventually defeated and killed on Lorn V by the Ork Warboss Gorgutz who cut off the Chaos Lord's head.
Another outcome takes place if the Word Bearers Dark Apostle Eliphas the Inheritor successfully destroys the Ork faction in the Dark Crusade on the world of Kronus in Dawn of War - Dark Crusade. Eliphas recovers Crull's skull and adds it to the tally for the Skull Throne of Khorne. Rather than the Manreaper wielded by the Chaos Lord Bale in the original Dawn of War, Crull was armed with a large Khornate mace.