Check out Ifan ben-Mezd's companion quest description:
Stage 1: Fort Joy
The quest starts right after you recruit Ifan ben-Mezd to your party.
Search his inventory and find a letter. From it you can learn more about this character.
Then, you must go to magister Borris and allow Ifan to speak with him. He orders you to find Zaleksar. If Ifan ben-Mezd is your main character then, in order to get information about Zaleksar, you have to kill Saheila or Borris.
Zaleksar is in Hollow Marshes near a destroyed bridge.
During this conversation you learn that Ifan's goal is Alexandar himself. To help you a little bit, Zaleksar will offer you a crossbow.
Here, this quest coincides with the main storyline because you need to kill Alexandar in order to leave the island.
Stage 2: Reaper's Coast
After that you reach Reaper's Coast. A boy will talk to you near a gate that leads to Driftwood. He informs you that Baran waits for Ifan in The Black Bull.
Go there and speak with Baran. He informs you that Roost wants to see Ifan in Sawmill.
Roost can be found on a floor of one of the buildings in Sawmill.
If Saheila survived the events on Fort Joy then you will find her imprisoned here. Let Ifan speak with Roost. You learn that Alexandar is still alive. He also asks Ifan to turn against your party. However, Ifan won't do that. This provokes a fight. Search Roost's corpse and collect Divine's orders.