Fossegrim

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"Hey kid, wanna learn how to play the fiddle?"

Fossegrim ("Waterfall Spirit" in Norwegian) or Strömkarlen ("The River Man" in Swedish) are fey spirits or trolls from Scandinavian folklore associated with rivers and waterfalls. They are depicted as male spirits, looking either like incredibly handsome young men, ugly fish-men, or somewhere in-between. Traditionally, they have an incredible knack for playing the fiddle, though most older art depicts them with harps instead, because fiddles aren't classical enough. They have such skill with the instrument that it can be used to work magic; 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.

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, or smoked mutton stolen from the neighbour's storage four Thursdays in a row. If there is not enough meat on the bone, he will only teach the supplicant how to tune the fiddle. 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".

Dungeons & Dragons

Fossergrim (with an R) is the Dungeons & Dragons version of this fey, appearing in the Fiend Folio for Dungeons & Dragons 3rd Edition. Taking the form of handsome human males with long white hair, blue-tinged white beards, and piercing blue eyes, typically wearing beautiful, fine-quality chain mail, and weapons, and nice clothing and exquisite silver jewelry, they are portrayed as essentially a masculine equivalent to the naiad. Fossergrims are shy and reclusive, generally leaving people in peace unless they actively try to pollute or dam the river to which it is connected. They do like to seduce beautiful maidens, however, who give birth to fossergrim sons.

Pathfinder

Pathfinder fossegrims play the harp, and are basically masculine, inland sirens: luring people in with music and illusions of treasure, only to drown them underneath their waterfall.