He has served the Imperium faithfully in his many years of service, however Inquisition are always watchful of their agents, especially one's who are the sole survivors of their last mission.
At the Beginning of Exterminatus, he has been sent on the orders of the Inquisition to the planet Magnum Christi, where previously the Thousand Sons traitor legion had once been in control. In their time there they had enslaved the population of the planet, using them as a labour force to excavate several Chaos Titan's. Having since been liberated by the Black Templar Space Marine's loyal to the Emperor.
Inquisitor Alastor's mission is to investigate the planet Magnum Christi and it's population, for signs of corruption and to choose the best course of action. Upon landing on the planet he is met by Chastener T'Sera of the Magnum Christi Adeptus Arbites, she requests to help him in anyway she can to prove her loyalty to the Emperor and quell any question of corruption and tainted of the ruinous powers. After seeing the Chaos Titan's for himself, Alastor is attacked by an abomination part biological, part mechanical and designed by Chaos. At the same time he is under unfamiliarly strong negative psychic attack, seeing that it emanates from a girl strapped into a machine, destroys her and the machine, then turning his focuses on the monster. After swiftly taking it, Alastor starts to understand the threat this planet and the technology found presents. That the girl was null psyker able to clock the Titan's from the Imperium gaze with the help machine amplifying her powers. They expand their search and find dozens more Titan's buried beneath the ground. Alastor then decide to take Chastener T'Sera as his companion and orders an Exterminatus on the planet Magnum Christi, fearing it too far gone for redemption.