Look for Wulf's Brother Torkild
After freeing the Skaal Village from Miraak's influence (see: The Fate of the Skaal), approach the hunter named Wulf Wild-Blood (he's usually forging weapons by his workshop). The man will tell you that his brother has gone missing some time ago and the locals claim that the poor man has become a werewolf. Regardless of his fate, you should head out to find him.
Torkild (that's his name) can be found in random places - that's why you can have some troubles with locating him. During your travels, look for small groups of half-naked men - the boy will be amongst them. Upon meeting with him, he will attack you (probably changing to a werewolf). Kill him - unfortunately it's unavoidable - and search his corpse.
Take a look at the corpse and take the Letter to Wulf. Read it to learn that he has found an ancient and mysterious power in the wilderness of Solstheim, a power which apparently was too much for him to bare. The change caused him to become a lunatic, which you could have experienced a moment ago. There's nothing left but to return to the quest giver and tell him the sad news.
Return to Wulf
Return to the Skaal Village and tell Wulf about the fate of his brother. Hand him over the letter and explain the situation. In return, he will teach you what he knows about surviving in the wilderness, that is increase your combat skills by one point (Archery, Block, One-handed, Two-handed and Heavy Armor). As you can see, the reward is worth the trouble.