The Loathly Lady (Motif D732 in Stith Thompson index), is an archetype commonly used in medieval literature, most famously in Geoffrey Chaucer's The Wife of Bath's Tale.
The motif is that of a woman that appears unattractive (ugly, loathly) but undergoes a transformation upon being approached by a man in spite of her unattractiveness, becoming extremely desirable. It is then revealed that her ugliness was the result of a curse which was broken by the hero's action.
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 the Fenian Cycle of Irish mythology, Diarmuid Ua Duibhne was one of the most famous members of the Fianna.
One freezing winter's night, the Loathly Lady brazenly entered the Fianna lodge, where the warriors had just gone to bed after a hunting expedition. Drenched to the bone, her sodden hair was snarled and knotted. Desperate for warmth and shelter, she knelt beside each warrior and demanded a blanket, beginning with their leader Fionn. Despite her rants and temper tantrums, the tired men only rolled over and ignored her in the hope that she would leave. Only young Diarmuid, whose bed was nearest to the fireplace, took pity on the wretched woman, giving her his bed and blanket. The Loathly Lady noticed Diarmuid's love spot and said that she had wandered the world alone for 7 years. Diarmuid reassured her and told her she could sleep all night and that he would protect her. Towards dawn, he became aware that she had become a beautiful young woman.