Thorn in the Side is a quest started by Tilian, found outside the small encampment, just south of the Champion Harbour. You've got only one shot at accepting the quest, Tilian will initiate the conversation himself and pull you in the shady business. Titian would like Saul killed, the prisoner they've recently caged for killing a Slayer. If you chose Kenneth to be your Skill Trainer, you will receive the quest Apprentice and Adversary in which case you'll want to save Saul and fail this quest.