Story question

[Smoketest]

He is a "vehicle" for telling the story. How he ended up having dreams of the wanderer isn't explained, and how Tyrael expected that whelp of a man to actually reach the hellforge, much less wrest the hammer from Hephasto, is beyond me. However, he does provide a convenient means for Blizzard to tell the story, at least in the cinematics, without having the main character present.

As a side note, Deckard Cain refers to Marius in one of his Act 5 gossip dialogs:

It is my belief that the soulstones are at the center of this conflict. If only that fool Marius had not intervened, Baal would still be imprisoned within Tal Rasha.