The Loathly Lady was a common figure in medieval literature; her most famous appearance is probably in Chaucer’s The Wife of Bath’s Tale.
In that story, a knight of King Arthur’s court rapes a woman. He is to be executed, but Guinevere intervenes. He will be spared if he can learn what it is…
Another Loathly Lady appears in Child ballad 32, “King Henry.” This time there’s no rape involved. The British folk rock band Steeleye Span (I adore them) recorded a version.
Kirkwall Was Totally Built On a Hellmouth 