Diablo III Development is Official
As for the story: "The game takes place on Sanctuary, a world of dark fantasy. Unbeknownst to most of its inhabitants, Sanctuary was saved some twenty years ago from the demonic forces of the underworld by a few brave and powerful heroes. Most of those warriors who directly faced the armies of the Burning Hells – and were fortunate enough to survive – went mad from their experiences. And most of the others have buried their haunted memories and pushed the horrors from their thoughts. In Diablo III, players will return to Sanctuary to confront evil in its many forms once again."