An Unpaid Debt - Skellige
Location: Strange in a Strange Land/Kaer Trolde Harbor
Quest Giver: Simun
Other Requirements: Accept Simun's help during Stranger in a Strange Land
This quest is technically begun during Stranger in a Strange Land but isn't actually started until after that quest is done and a few days pass in-game. While doing Stranger in a Strange Land if you accept the help from Simun Brambling to leave the jail cell then you'll be able to do this quest. If you did not accept his help then this quest will be unavailable and you'll never be able to complete it.
After you complete Stranger in a Strange Land as The Cave of Dreams quests you'll be able to find Simun outside of the Kaer Trolde Tavern, which is located in the center of town (pictured below). Speak with him to accept the quest An Unpaid Debt.
Simun asks you to meet him near a lake southwest of Miners' Camp Signpost - head down here to find him arguing with some man at his house. Simun will ask you to kill the man, no questions asked.... Pretty steep price for a favor, huh? If you hesitate then Simun will attack you instead and you'll fail the quest.
The only way to complete this quest is to help Simun and kill the guy.