Act III: The protagonist travels to Bastion’s Keep with Leah, their followers, Tyrael and Adria only to find it by the military of Azmodan. Tyrael ruins it, and helps the protagonist in reaching the demonic gate shielding the crater. Following a ghastly conflict having a tremendous Siegebreaker Assault Beast, the protagonist traverses the depths of the interior center, ruining the Sin Hearts in Arreat.
The protagonist, who subsequently faces and conquers Azmodan, tries to defend the hearts, but defeats the Maiden of Lust, Azmodan’s consort Cydaea. Leah seals the soul inside the Black Soulstone of Azmodan. With all seven Lords of Hell immobilized inside the rock, Tyrael states the Endless Struggle between Hell and Heaven will soon be stopped in the event the rock is ruined.
The protagonist returns to the Keep of Bastion, but finds that they have been betrayed by Adria. Adria uses while driving Leah to function as his boat, the Black Soulstone to resurrect Diablo. With the spirits of the Lords of Hell within him, Diablo starts an assault, and becomes the Prime Evil.