Overview
Farting Trolls is an unmarked secondary quest in The Witcher 3: Wild Hunt, set in an unmarked cave on the road between Arinbjorn and Kaer Muire, just west of the place of power there, on Ard Skellig in the Skellige Isles.
What happens
As soon as Geralt enters the cave, the name of the quest explains itself. Deeper in, three rock trolls are chatting and breaking wind in equal measure. They are not friendly and attack if they notice him, so to explore the cave you have to kill them. That completes the quest.
A chest near the boat in the cave holds the Enhanced Ursine boots diagram.