Flesh for Sale - Skellige
Quest Giver: Acquired when entering or approaching Trottheim
Other Requirements: Haven't completed Following The Thread
Note: This quest will become unavailable after the step Speak with Lambert at the Seven Cats Inn in Following The Thread has been completed. If you plan to do this quest do it before you complete that one.
To begin this quest you'll need to travel to the town of Trottheim in Skellige which is currently occupied by pirates. It's on an island in the southeastern most portion of the map, if you're having trouble locating it use my screen shot below for additional guidance.
For this quest, if you approach either of the guarded entrances to the town you'll be confused for a Nilfgaardian and escorted to the pirate's boss. You can keep up the rouse the entire time and get a key for the slave cells and free the prisoners, then kill the pirates if you wish.
Alternatively if you enter the town any other way the pirates will immediately attack you. You can also give up the rouse at any time and just kill all of the pirates in the town if you wish, the outcome of the quest will be the same either way.
If you decide to keep up the rouse the entire time Geralt will convince the pirate's boss to give him the key to open the slave cell. Otherwise you'll loot the key from his corpse and free the slaves that way. One of the slaves will give you a Diagram: Ban Ard Breastplate and the quest will conclude here.