This page of the guide to The Witcher 3: Wild Hunt includes a walkthrough of the only witcher contract in Hindarsfjall . The following walkthrough suggests how to find out about the beast of Hindarsfjall, how to track down nekkers, and how to defeat Hagumban in the cave.
Contract: Strange Beast
You will learn about this contract from the notice board in Larvik island (M18,1). To receive more information, go to Thorleif (M18,2) who can be found on the other side of the village. You won't receive much valuable information, but remember to bargain the reward size. Now go talk to Sverr who also can be found in the village.
After the conversation, go to the place where warriors got ambushed. Search it with the witcher senses. Once you do it, you will learn that nekkers with a very intelligent leader were responsible. Now find the footprints and follow them. After over a dozen meters, you will reach a cave. Be careful as you walk through it, there are many poisonous fumes inside.
Keep going through the tunnel until you reach the monster's nest. Kill three regular nekkers on the 13th experience level. After a while their leader will show - Hagumban. He is a standard nekkers, only with much more health. Watch out for a groups of three nekkers on the 13th level appearing from time to time. From the body of fallen enemy you can take alchemy ingredients, trophy and Devine sword. Now go back to the quest giver for the reward.
The main reward for completing the quest is a nekker trophy : + 5% chance of finding additional herbs.