Urcheon of Elderwald's story refers to The Brothers Grimm fairy tales, in which a similar story of a hedgehog boy and the Law of Surprise can be found in the tale of "Hans my Hedgehog".
Largely based on the short-story "A Question of Price" from the book "The Last Wish".
The selkiemore monster that Geralt kills in this episode (offscreen) is specific just to the show. It does not exist in the game.