Harpans kraft (Swedish) or Harpens kraft (Danish), meaning "The Power of the Harp," is the title of a supernatural ballad type, attested in Danish, Swedish, Norwegian, and Icelandic examples.
In The Types of the Scandinavian Medieval Ballad it is catalogued type A 50, "Man saves his bride from merman by playing his harp."
The ballad type tells of a hero whose betrothed has premonitions of a fall from a bridge into the river, which despite the hero's assurances and precautions comes true. But by the power of his harp-playing, he regains his bride from the river creature, which is referred to as a "merman" in the TSB catalog, but more precisely given as "neck (water spirit)" or troll in the original Scandinavian texts.
The ballad of this type occur under the following titles. Danish: "Harpens kraft" (DgF 40); Swedish: "Harpans kraft" (SMB 22); Norwegian: Villemann og Magnhild (NMB 26); Gaute og Magnild and Guðmund og Signelita (Landstad 51 and 52), etc.; and Icelandic: Gautakvæði "Gauti's ballad".
Noted for its resemblance to the Greek myth of Orpheus, a harp-player with mystical powers, it may be related to medieval versions of that story such as the Middle English Sir Orfeo.
Similarity has also been noted with the supernatural power of the harp in the Scottish ballad Glasgerion (Child ballad 67 variants B, C, "He'd harpit a fish out o saut water," etc.).
A bridegroom asks his betrothed why she is so sorrowful. At last she answers that she is going to fall into a river on her way to her wedding (as her sisters have done before her, in some Danish, Norwegian, and Swedish variants). The man promises to build a broad, strong bridge over the river, and he and his men will protect her. Despite precautions, the maiden's horse stumbles (or rears up) while over the bridge, and she tumbles into the river. The man has his golden harp brought to him and plays so beautifully that the "merman" (Danish: trold; Swedish: neck i.e. "neck (water spirit)") is forced to return his betrothed.