Severnius was an Ordo Xenos Inquisitor who served as the mentor and first commander to famed Deathwatch member Artemis. Serving alongside Artemis for decades, he proved essential in a battle against a dangerous race of Xenos on the Agri-World of Tarrenhorst. Since these xenos were able to psychically dominate their victims, Severnius's own psychic abilities were the only thing which saved his Kill-team. However, over the course of a desperate six-day battle, Severnius was forced to maintain this shield, a process which exhausted him. Just before they were rescued by Thunderhawk gunships, a weakened Severnius was struck down by a psychically enslaved Space Marine, thus passing command of the Kill-Team to Artemis.