Ungersven och havsfrun (as it is known in Swedish) or Herr Bøsmer i elvehjem (as it is known in Danish) (The Types of the Scandinavian Medieval Ballad A 49, Drink causes forgetfulness and makes man stay with mermaid) is a supernatural ballad type.
In most variants, a mermaid comes to a young knight and while he is asleep she asks him to come to her home in the river (or sea).
In most variants he receives a drink which either makes him forget his family and fiancée and stay with the mermaid.
In others he escapes by playing his harp:[3] in Swedish variant G (from Brita Cajsa Carlsdotter from Östergötland, collected by L. Chr.
Wiede in the 1840s) the young man saves himself by playing beautifully on his golden harp, rather like in the ballad Harpans kraft.