Talk to Grigor(location: The Sarn Encampment, Act 3)
Objective
Kill Dominus(location: The Upper Sceptre of God)
Completion
Talk to Lady Dialla(Upon defeating Dominus, Lady Dialla appears to congratulate the player and opens the way to act 4)
Sceptre of God walkthrough
1. Unlock The Sceptre of God
After completing the Piety’s Pets quest, you can get Tower Key. Use the key to open the locked door found in the Imperial Gardens.
2. Find Dominus
Dominus is the final boss of act 3, located in The Upper Sceptre of God, on the Tower Rooftop (after climbing 3 or 4 staircases in the Upper Sceptre of God).