Marius was a drifter who witnessed the horrors of Tristam, and ends up helping the Prime Evils to a great extent before being killed by Baal.
Background
After Marius witnessed the horrors of Tristam and followed the Dark Wanderer, he then becomes the Wanderer's companion. Marius' sanity is questionable, as well as his courage. He is tricked into removing Baal's soulstone and then is unable to go through the infernal gate to Hell to destroy the soulstone at the Hellforge.
Some time later, Baal visits Marius, in human form in his cell in a mental asylum. Marius then narrates the events of Diablo II to Baal, who he mistakes for Tyrael. In the end, Marius confesses that he did not destroy the soulstone and gives it unwittingly to Baal to be destroyed at the Hellforge. Baal then reveals himself and kills Marius, destroying the asylum in the process.
Canonity and Time
Marius is depicted as a very old man by the time Baal comes to visit him, implying there has been many years between the end of Diablo II and the beginning of the expansion (Diablo II: Lord of Destruction). In the new timeline (and even just in the Diablo II story line) this does not fit in. Either just holding Baal's soulstone for a few weeks/months have aged him severely, or this is a timing error by Blizzard North.
Regardless, the canon information is that it only took a brief while between the death of Diablo and the events in the unnamed mental asylum and meeting Baal.