Durak Rask was a fanatical follower of his Primarch, and viewed Mortarion as a preternatural saviour from his early youth on the benighted world of Barbarus. Having first been accepted into training into the Death Guard Legion when he came of age, Rask's innate intelligence and fervour were marked from an early stage and he quickly proved to be a dour and driven warrior with an uncommon flair for siegecraft. Over his decades of service in the XIV Legion, Rask rose steadily through the ranks to become his Legion's Marshal of Ordnance, proving his mettle in many battles and campaigns, and earning a rare commendation from Mortarion himself after the Siege of Valstpol, a battle in which Rask himself lost his left eye and was left horrifically scarred. When Mortarion declared his allegiance for Horus at the start of the Horus Heresy, Rask was one of the first and loudest in his support, having already become a member of the Seven Pillared Warrior Lodge which had been founded within the Death Guard to corrupt them towards the service of Chaos from within. On Istvaan III Rask volunteered to lead the vanguard attack against his former Battle-Brothers, a treachery he would pay for with his life.