Overview
Monster Slayer is a secondary quest in The Witcher 3: Wild Hunt, set near Lakeside Hut in the valley around Kaer Morhen.
What happens
To the right of Lakeside Hut lies a dead drowner. Examine it and keep following the bodies until a cave opens up on the right. On the edge of the small island just before the cave lies the Enhanced Wolven trousers diagram.
Inside the cave, an ancient rock troll attacks. After it falls, Witcher Senses reveal an old letter describing an original plan to attack Kaer Morhen.