The Sad Tale of the Grossbart Brothers
Side quest
Skellige suggested level 26 
On the world map
From the journal
If ever I tire of writing of Geralt of Rivia, I will take up the tale of Djenge Frett, an itinerant bounty hunter. Geralt met this extraordinary man in Skellige, where he had gone to hunt the notorious Grossbart brothers. According to Frett's information, they were currently up to some nefarious business in the southern part of the isle of Ard Skellig.
The job was done. The world was finally free of those ruthless and bloodthirsty brothers. And it was all due to the brave Djenge Frett - and my dear friend, Geralt of Rivia.
Djenge Frett's information was correct. The Grossbart brothers truly were hiding on Ard Skellig. Geralt and the bounty hunter decided to take them out together.