The Nithing is a side quest in Skellige that can be picked up from the Rannvaig notice board.

After picking up the quest, talk to Lothar about more information on the contract. After talking to Lothar, use your witcher senses and investigate the nearby Nithing, and then the tracks leading away from the Nithing. You will eventually see a womans shawl, with a distinctive scent. Follow the scent using Witcher Senses.

You will eventually arrive at the nearly village. Wait in the village until day time and ask villagers about the shawl.

After talking to one of the women, she tells you the shawl belongs to Jonna, the herbalist. After talking to Jonna, she tells Geralt what she wants to lift the curse.

Head back to Lothar and tell him what you discovered. You have the option of letting Jonna die, or telling Lothar you can't do it.

Fix the curse on the Nithing, then return to Lothar to collect your reward.