The Scandinavian näck, näkki, nøkk, nøkken, strömkarl, Grim or Fosse-Grim were male water spirits who played enchanted songs on the violin, luring women and children to drown in lakes or streams.
It is difficult to describe the actual appearance of the nix, as one of his central attributes was thought to be shapeshifting. Perhaps he did not have any true shape. He could show himself as a man playing the violin in brooks and waterfalls (though often imagined as fair and naked today, in actual folklore he was more frequently wearing more or less elegant clothing) but also could appear to be treasure or various floating objects or as an animal—most commonly in the form of a “brook horse”. The modern Scandinavian names are derived from an Old Norse nykr, meaning “river horse.”