Xarl grew up with Talos Valcoran in the hive city of Nostramo Quintus on Nostramo and were members of the same street gang, fighting for survival on the unforgiving streets of the perpetual night world. Later, both friends were inducted into the VIIIth Legion, becoming fully-fledged Legionaries and served alongside one another in 1st Claw (squad), 10th Company during the Great Crusade and the Horus Heresy in the 30th and 31st Millennia. Xarl's skill with his double-handed Chainsword made him the deadliest warrior in the 10th Company, and gave Talos and 1st Claw much of its clout within the warband.