Loathly lady
The loathly lady is a common literary device used in medieval literature, most famously in Geoffrey Chaucer's The Wife of Bath's Tale. The motif was prominent in Celtic mythology and to a lesser extent Germanic mythology, where the lady often represented the sovereignty of the land.
Characteristics
The loathly lady is a hideous woman who demands that a man kiss or marry her in her hideous form. This action reveals that her shape is a transformation that has been broken.
Celtic tradition
The loathly lady can be found in The Adventures of the Sons of Eochaid Mugmedon, in which Niall of the Nine Hostages proves himself the rightful High King of Ireland by embracing her; the motif can also be found in stories of the earlier high kings Lugaid Loígde and Conn of the Hundred Battles. In her capacity as a quest-bringer, the loathly lady can be found in the literature of the Holy Grail, including Chrétien de Troyes' Perceval, the Story of the Grail, Wolfram von Eschenbach's Parzival, and the Welsh Romance Peredur, son of Efrawg associated with the Mabinogion.
The theme became a staple of Arthurian literature; the best known treatment is in the Wife of Bath's Tale, in which a knight, told that he can choose whether his bride is to be ugly yet faithful, or beautiful yet false, frees the lady from the form entirely by allowing her to choose for herself. A variation on this story is attached to Sir Gawain in the related romances The Wedding of Sir Gawain and Dame Ragnelle and The Marriage of Sir Gawain.
Germanic tradition
The loathly lady also appears in the Old Norse Hrólfr Kraki's saga where Hróarr's brother Helgi was visited one Yule by an ugly being while he was in his hunting house. No person in the entire kingdom allowed the being to enter the house, except Helgi. Later, the thing asked to sleep in his bed. Unwillingly he agreed, and as the thing got into the bed, it turned into an elvish woman, who was clad in silk and who was the most beautiful woman he had ever seen. He raped her, and made her pregnant with a daughter named Skuld. Helgi forgot the woman and a couple of days after the date had passed, he was visited by the woman, who had Skuld in her arms. The daughter would later marry Hjörvarðr, Hrólfr Kraki's killer.
This tradition is also present in the Northumbrian tale The Laidly Worm of Spindleston Heugh. Similar to this tale, is that of Hjálmþés saga ok Ölvis.