After the demonic brothers Diablo and Mephisto were finally defeated, the heroic adventurer who has accomplished that feat returned to the Pandemonium Fortress, summoned by Archangel Tyrael. The third evil brother, Baal, has survived by obtaining his Soulstone. He has raised an army and attacked Mount Arreat in the Barbarian Highlands. The protagonist must venture there and defeat Baal once and for all.