"Allison Gross" (also known as Alison Cross) is a traditional ballad, catalogued as Child Ballad #35. It tells the story of "the ugliest witch in the north country" who tries to persuade a man to become her lover and then punishes him by a transformation.

'Wiki'

"This tale is a variety of 'Beauty and the Beast'
An elf, a hill troll, a mermaid make a young man offers of splendid gifts to obtain his love or the promise of his faith"

From The English and Scottish Popular Ballads edt Francis James Child