After long years fighting as a space wolf, all served under the same wolf lord, Hagaar's squad left on a great hunt with the rest of their Great Company. They began winning great victories, but were caught flat footed by an Eldar raid as they were investigating a planet. Much of the company was wiped out, and during a desperate holding action trying to buy time for others to get back to a waiting thunderhawk, all of the surviving members of Hagaar's Long Fang pack were killed. Hagaar himself only survived after grevious wounds thanks to his Sus-an membrane taking him into suspended animation. Left for dead by the Eldar, he was discovered when the remnants of the Company scouted the battlefield later.
After spending much time in the care of the Wolf Priests recovering, the wolf lord offered him choices - he could return to the fang and train new recruits, or continue to serve the company as a wolf scout or lone wolf - but Hagaar requested secondment to the Deathwatch, to face the Xenos that had killed his brothers over the many years of his career, and avenge their deaths.