Fairytales in the world of Geralt aren't all that likely.
The Witcher 3: Blood and Wine - A Knight's Tales
This quest can be discovered on the Castel Ravello Vineyard noticeboard or can simply be stumbled upon by finding its starting point marked by an exclamation point on the map. It's suitable for Level 40 and up.
The chap who posted the notice, Jacob, is hanging out next to a tree. He tells you of his theory that there's something off about this tree. You'll have to use your Witcher senses to investigate, and it doesn't take Geralt long to conclude that, yes, there's something rather amiss here.
Talk to Jacob again and accept some items off him. Read the book. Now you have a new objective. Head there - you'll have to disembark Roach at a point and clamber up on foot until you reach a Witch's hut. Examine things in here with your Witcher senses, then the hut's owner appears. At this point you can deal with her in a number of ways - threats, charm, it's your choice.
If you charm her, you'll need to head to a cave not far from the Witch's hut to retrieve a bone, plus the handkerchief from the hut. If you threaten her you'll end up in a fight to force her to capitulate. Either way, at the end, return to the tree.
If you were nice, you'll have a ritual to perform in order to lift the curse. If you were nasty... things take another twist. Whatever happened, this quest will now be complete.