"Clerk Colvill" is Child ballad 42.
This ballad was one of 25 traditional works included in Ballads Weird and Wonderful (1912) and illustrated by Vernon Hill.
Clerk Colvill, ignoring the advice of his lady or his mother, goes to a body of water, where a mermaid seduces him. His head starts to ache, and the mermaid tells him he will die of it. He goes home and dies.
In some variants, she offers that he may go to sea with her instead of dying, at the end, and he refuses.
Francis James Child regarded the ballad as incomplete, and that Clerk Colvill is not an innocent victim of jealousy, but has clearly had a relationship with the mermaid, so that she inflicts death as the penalty for his infidelity, a motif found in many German and Scandavian tales.
Similar ballads exist in the Nordic countries. The ballad is called "Elveskud" in Danish, "Olav Liljekrans" in Norwegian and "Herr Olof och älvorna" in Swedish and there is also Faroese and Icelandic versions. In these ballads the role of the mermaid is taken by an elf woman.