Fossegrim

Fossegrim, also known simply as the grim (Norwegian) or Strömkarlen (Swedish), is a water spirit or troll in Scandinavian folklore.

The Swedish strömkarl's lay is said to have eleven variations, the final one being reserved for the night spirits because when it is played, "tables and benches, cup and can, gray-beards and grandmothers, blind and lame, even babes in the cradle" will begin to dance.

[4] Fossegrim is said to be willing to teach away his skills in exchange for a food offering made on a Thursday evening and in secrecy: a white he-goat thrown with head turned away into a waterfall that flows northwards,[2][5] or smoked mutton (fenalår).

If the offering is satisfactory, he will take the pupil's right hand and draw the fingers along the strings until they all bleed, after which he will be able to play so well that "the trees shall dance and torrents in their fall stand still".

[2][5] Jacob Grimm cites a variant in 18th-century Chorographia Bahusiensis by Johan Ödman (1682-1749) according to which the Strömkarlen must be offered redemption or he will merely break his instrument and weep bitterly.