The game takes place in the dark fantasy world of Sanctuary, where the forces of evil have returned to threaten the land. The story follows the Nephalem, a hero tasked with defeating the Prime Evils, Diablo, Mephisto, and Baal, and their lieutenants, the Lords of Destruction, Azmodan and Belial. The expansion pack introduces two new acts, Act V and Act VI, which conclude the story of the original game.