Like any self-respecting Blizzard game, Diablo IIIhas a massive amount of lore traditionally left out of the primary gameplay experience. To get some of the backstory of how Tristram got embroiled in this whole mess with the devil, check out this wonderful short film from Titmouse, the animation house of Peter Chung of Aeon Flux and Metalocalypse fame. It shows a bit about that whole struggle between hell and earth Deckard Cain is always talking about, and the fateful wrath that the angels fall prey to when first duking it out with the Lord of Darkness.
Thanks a lot, angels. Now we’ve got to pick up the mess.