Quoted from the Diablo 3 FAQ
He's the new Diablo!It has been twenty years since the events of Diablo II. Of those who faced the Lords of Hatred, Terror, and Destruction in the battle over the fate of Sanctuary, there are few still living who can bear to remember the horrors that the Prime Evils wrought upon the world. And of those who did not witness the terrible events firsthand, most believe the stories to be little more than myth. But something evil is stirring once again in Tristram, and it may already have claimed its first victim: Deckard Cain.