You mean " In Wolf's Clothing"? It automatically started during the main quest "Nameless" for me. First time, I killed it after feeding him rotten flesh. But he respawned. In order to permanently cure the curse you'll have to speak to Einar, then he'll give you a fang, hand it over to Morkvarg, afterwards cured Morkvarg will direct you to a guy in Novigrad for a reward.