Is a Velen side quest with suggested level 20.

Talking to the quest giver about the contract about your reward and you can increase the amount to at least another 30 crowns.

Head to the map marker, and examine the area with your witcher senses.

The first location will be lots of dead wolves that were taken out by the monster. Use witcher sesnes to find and then follow the path the monster took.

When you lose the trail, examine a near by tree to find the monster marking the tree. From this tree, there will be a scent which you can now follow.

You'll eventually come across some old and dried out fiend droppings, and the scent going up a path which you are unable to jump to. Find another way around and into the cave.

Once in the cave, examine the already dead Fiend. Examine the area further with witcher senses to determine what monster killed the Fiend. Also examine the Fiend's mouth to find fur from the other monster.

It turns out the monster is Howler which is a Chort. You will need to use alchemy to craft "Chort Lure." The ingredients are actually all around you, including Fiend Dung. Equip the lure when you've crafted it. Once it is equipped, exit the cave, and spray the bait around the hill in designated locations.

After the bait has been spread around, head back to the cave and wait for the monster to appear.

After defeating the monster, head back to the quest giver to receive your reward.