The first Diablo manual had great background stories and mythology. When I watched Constantine for the first time, I realize it had the same setting as Diablo!
They reuse some religious themes etc, but other than that they are not related what so ever...
I mean the whole idea of heaven and hell fighting for power and that they are not supposed to have any physical influence, but the demons of hell sneak in anyways, not just by mental influence but physically as well. That's how the Diablo 1 story begins, with the corruption of the Archbishop (I remember way too much lol).