Much of Arktus’ background is shrouded in mystery. The only person he seems to trust is Malcolm Lancaster- and he only half trusts the dragon priest, at that.
It is known that Arktus hails from Iberia, and had been traveling in Agraba when he met Malcolm. It is also known that he betrayed former masters to help Malcolm. It is also known that he is as talented with dark magic as he is with his massive greatsword.
Whatever else is left unknown, it is clear that Arktus has a dark story to match his dark demeanor.