Talk to Eramir(location: The Bridge Encampment, Act 7)
Objective
Defeat Ralakesh(location: The Ashen Fields)
Completion
Talk to Eramir
Kill Greust to defeat Ralakesh
Location: The Ashen Fields
In the boss fight Ralakesh is not targatable and aiding Greust. Killing Greust will end the boss fight. Sin will then kill Ralakesh by draining its power.