Marius was a bartender/waiter in Tristam. He follows the Dark Wanderer after the destruction of Tristam. He then becomes a companion to the Wanderer. He is not too bright, as he is responsible for the freeing of Baal and then fails to go to Hell to destroy Baal's soulstone.
Many years later, Baal visits Marius, now an old man, in human form in his cell in a mental asylum. He then narrates the events of Diablo II to Ball, who he mistakes for Tyrael. In the end, he confesses to Tyreal that he did not destroy the soulstone and gives it to Baal for destroying it at the Hellforge. Baal then reveals himself and kills Marius, destroying the asylum in the process.