Hanna's sister Margaret knew this secret, and had one of her own - she was in love with Niellen and fiercely jealous of Hanna's life with him. Afflicted with the curse of lycanthropy, he would hide in the forest during his transformations in order to protect his friends and loved ones. It turned out she had been killed by her own husband. The story of Hanna's disappearance had a tragic end. If you don't interfere and Niellen will bring Margrit justice, then when you leave the cave, Niellen will catch up with you one more time and ask you to end his miserable fate. If you don't let a werewolf kill Margaret, then you' ll have to finish him. It's up to you whether you let him do it or not.

Angry Niellen will decide to punish Margrit for what she did and kill her. It will turn out that Niellen is a werewolf and Margaret is responsible for the death of his wife. When you're about to kill him, Margaret will come to the cave and stop the fight. Wait (meditate) till nightfall and a werewolf should appear in the cave.

Get into the cave and walk down to the bottom of it. You can destroy the wooden door with an aard sign.

Witcher 3 wild at heart werewolf heals