Castillon hired Isabela, the captain of the Siren's Call to transport slaves. The slaves were Fereldan refugees fleeing the Fifth Blight that paid Castillon for transport. He took their money and turned them into slaves. When Isabela disobeys his orders and frees the slaves, he tasks her to retrieve the Tome of Koslun as a way of paying back and Hayder to track her progress.
In order to meet Castillon, one must first convince Isabela near the end of Act 2, in order to have her come back with the Tome of Koslun as well in the final battle with the Arishok. Only by doing so will the player access Isabela's final companion quest.
In Act 3 during the quest No Rest for the Wicked, Hawke and Isabela plan to find Castillon using one of his men. Allow Isabela to willingly turn herself in and have the Champion follow her trail, leading Hawke to a warehouse in the Docks. Once there, they kill everyone in the warehouse and await Castillon's arrival. While waiting, Isabela advises searching through the warehouse for leverage and the group finds documents outlining Castillon's plans to expand his slaving organisation to the Free Marches. Castillon soon arrives and Isabela will threaten to reveal Castillon's slavery ring plans to the Kirkwall authorities. She offers him an ultimatum: She will give the documents back to Castillon on the condition that he will leave his ship into her command and swear never to hunt her down again. Hawke can either agree with Isabela's wishes or demand Castillon die for his crimes.
If you bring Aveline or Fenris along during this quest and choose the option to make Isabela give him documents while letting him go, you will receive (+10) from both of them. If you have Sebastian, you will receive (+5).Conversely, if you choose to kill Castillon, there is no change in approval.
Requires: 29 dexterity
+145 attack – according to the official guide. As of patch 1.04, it is not received, likely due to a bug.