After defeating the demon Ghom, the Lord of Gluttony, the protagonist travels to crater of Mount Arreat to confront Azmodan. Tyrael assists the protagonist in reaching the demonic gate protecting the crater, and destroys it with his sword.
The protagonist traverses the depths of Arreat's inner core to destroy the Sin Hearts, which empower Azmodan and his armies. Azmodan's consort Cydaea, the Maiden of Lust, attempts to protect the hearts but is defeated. The protagonist confronts and defeats Azmodan, and Leah seals his soul in the black soulstone. With all seven lords of hell trapped within the stone, Tyrael states that the war will be ended if the stone is destroyed. The protagonist returns to Bastion's Keep, but finds that Adria has betrayed them.
Adria reveals she has been serving Diablo from the beginning, and that Leah's father is Leoric's son Aidan, the Dark Wanderer (the Warrior player character from the first game and Diablo's reincarnate body in Diablo II), who was possessed by Diablo.