The Nøkken, or Näcken, is a dangerous creature from Scandinavian folklore. They are water-dwelling shapeshifters who usually appear in the shape of a naked man playing a violin or similar instrument, or a beautiful white horse. They lure unwary travelers into the water and drown them.
These creatures are sometimes called Fossegrim, but this name is applied almost solely to the most harmless of these water spirits. Stories exist in which a Fossegrim agrees to live with a human who fell in love with them or agree to help travelers, for a price.
Similar creatures exist in all Germanic mythologies.