A Rare Prey quest
Stage 1: Fort Joy
There are two moments in the game that allow you to start this quest. The first one is a conversation with Gareth in Sanctuary Of Amadia. He will tell you about Godwoken. You also learn that Alexandar is one of them.
The second way is to meet with Windego. She is located on the beach near the port. Regardless of how the conversation goes, you will have to face her.
The main objective of this part of the quest is to kill Alexandar. This is connected with a main quest from The Escape chapter.
Stage 2: Reaper's Coast
A boy can talk to you near a gate that leads to Driftwood but this can only happen if Ifan ben-Mezd is in your party. The boy orders Ifan to go to The Black Bull where Baran is waiting for him.
Go there. You learn that someone ordered Lone Wolf to kill Godwoken. He also informs you that Roost is waiting for you in Sawmill.
Head to Sawmill. Having Ifan in your party allows you to enter the camp without any fights. Otherwise, you will have to break through with force.
Roost can be found on a floor of one of the buildings in Sawmill. Talk to him. The only interesting thing that you can learn from him is that Alexandar is still alive. The conversation ends with a fight. After that you must search Roost's and find Divine's orders.
You should also examine a Black Mirror that stands in a corner.
Interacting with it allows you to speak with Sallow Man.
Stage 3: The Nameless Isle
After reaching the land in Temple Of Rhalic you will witness a fight between Black Rings, paladins and magisters. You must support one of the sides.
Then, you must head to Temple Of Tir-Cendelius. You meet Alexandar and a group of his worshippers.
He can help you reach The Academy but, in exchange, he wants you to kill Sallow Man. You can agree or you can decide to kill him and take his head to Sallow Man.
Sallow Man hides in The Cave which is near Temple Of Rhalic. The entrance is guarded by a troll. You can convince him to let you in or you can kill the creature. If you selected the second option then you should use Restoration ability on him to remove Troll Blood status.
Enter the cave and reach a small room with an altar.
Sallow Man's hideout is behind an illusion. If you killed Alexandar then you can place his head on the altar. Otherwise, you must use Cowl Of The True Sight that you got from Alexandar.
The quest ends when you confront Sallow Man. It doesn't matter if you kill him or not.