The Sad Tale of the Grossbard Brothers is a Skellige side quest and is a bit of an easter egg as it has legendary bounty Djenge Frett which is a bit of a nod towards Star Wars' Jango Fett. Has suggested level 26, but you only need to deal with three fairly weak human enemies, so could be done at lower levels relatively easily.

You can pick up this quest from Djenge Frett himself, in front of the Blandare signpost.

The location of the Grossbart Brothers is located east of Ruined Inn, which is east of Fyresdal, and along the coast, and inside a cave.

You'll meet Djenge Frett inside before encountering the Grossbart Brothers. Killing the brothers will complete the quest.