(Middle Danish: næk; Old Norse: nykr)
In Nordic mythology and folklore, nøkken (the "-(e)n" indicates the definite article) is a male supernatural creature, which lives in a stream or river. In Danish folk tradition, he also sometimes called åmanden ("the river man").
Since the middle ages, Danish folk tradition has had it that the nøkke calls for human sacrifice with the words "Tiden er kommen, men manden er endnu ikke kommen" ("The time has come, but the man has not yet come"). In Norway and Sweden, there is a rich tradition of folkloric tales about the nøkke (in Sweden, näcken) as a musician. According to these tales, nøkken teaches others to play music (often, the fiddle) on condition of sacrifice - either of themselves, or of some sort of edible offering. One story tells of a boy who went to the nøkke with a ham, which he wanted to trade for the ability to play the fiddle. Along the way, he became hungry, and ate part of the ham. In return for this slight, the nøkke taught him to play, but not to stop playing, and the unfortunate boy played until his fingers bled.