Right after touching the Eye of God, you will turn around and be in the Void. Follow the left wall until you come across a lootable body. Loot him to gain a new quest marker. Follow the marker to his secret room and open the safe using the code "962" then interact with its contents. This should progress the quest marker. Now, you just need to reach the Outsider and say that you won't kill him. Talk to Daud and go through the dialogue saying that you won't kill the Outsider (you can repeat the dialogue if it doesn't trigger the non-violent ending).
Daud will read the Outsider's name, successfully returning him to the mortal world.