As Leah studies in Caldeum's library to find more answers about the Black Soulstone and Azmodan, she receives a vision from Azmodan, who tells her that he is sending an army from the ruins of Mount Arreat to take the Black Soulstone for himself.
Tyrael, Adria, Leah and the Nephalem journey to Bastion's Keep, the only line of defense between Azmodan's forces and the rest of Sanctuary.
Although the Nephalem retrieves the pieces, the witch Maghda seizes the shards and attempts to capture Cain to force him to repair the sword for her own ends.
However, with an uncontrolled display of power, Leah forces Maghda to flee, and she kidnaps the stranger instead.
With the others staying behind to protect the Black Soulstone, the Nephalem pushes out from the keep into Mount Arreat.
The Nephalem kills Azmodan and traps his soul in the Black Soulstone.
The Nephalem kills Belial and traps his soul within the Black Soulstone, freeing Caldeum.
Many more parts of what appear to be a seamless background environment are destructible.
Some dilapidated areas will also collapse (with or without harming the player) automatically when the player gets too near or passes under part of a structure.
After the battle, Tyrael decides to rejoin the High Heavens but remain as a mortal, dedicated to building a permanent alliance between angels and humans.
Diablo III incorporates the Havok physics engine and enables players to utilize the environment to help in their quest.