Harassing a Troll is a secondary quest in The Witcher 3: Wild Hunt.
Travel into the thick forested undergrowth heading northwest from the Inn at the Crossroads. Close to the river bank is a small collection of rocks, a campfire, and a larger collection of five nekkers intent on swarming over a hapless rock troll. Join in the fray, and slay all of the nekkers.
Afterwards, Boris informs you that his culinary tastes (nekker soup) may have influenced the attack. No matter, you are gifted a small prize (which could be anything from herbs, other junk items, or even an elf head, which elicits some additional surprising chatter from you) and the quest concludes.