Ragnelle, The Foul Ladye
In the medieval poem The Wedding of Sir Gawain and Dame Ragnelle, Arthur had fallen into the power of the Grim Baron, Gromer Somer Joure, who made him swear to return in a year and a day and either bring the correct answer to the riddle
What is it women love best?
or meet his death. While searching for the answer, Arthur met in Inglewood Forest a lady carrying a lute and riding a beautiful, richly caparisoned palfrey - but the woman herself was incredibly ugly and hideous. The Foul Lady was Dame Ragnell, Gromer's sister (though she did not tell Arthur that), and she gave Arthur the anwer to the riddle in return for his and Gawaine's promise that Gawaine would wed her.
Arthur returned to court and told Gawaine of his adventure, and Gawaine agreed to the marriage so that he could save Arthur's life. (The answer to the riddle is generally given as "Women most desire to have power over men", or "To have their own will in all things". I suspect the answer might just as well be stated, "Women most desire exactly the same thing that men most desire".) Gromer reluctantly sparing Arthur because of Ragnell's answer to the riddle, she came along to the King's court at Carlisle, insisting on her full rights.
Though Guenevere begged her to be married secretly and privately in the early morning, to spare Gawaine disgrace, Dame Ragnell calmly insisted on a full public ceremony with all the trimmings, and further set off her ugliness with a bridal dress worth three thousand gold pieces. At the marriage feast she enjoyed herself with hearty bad manners, gobbling down as much as any other six guests together. When, at last, she and Gawaine were alone in the bridal chamber, she demanded her marital rights, pointing out,
If I were beautiful, you wouldn't even have worried about whether we were married or not.
When he turned to give her what she asked, he beheld one of the most beautiful women he could ever have imagined. She then explained that he could choose whether to have her beautiful at night for himself alone and ugly by day in the sight of the world, or beautiful by day and ugly by night. Stymied, or at least pretending to be, he gave the choice back to her. His generosity broke the enchantment completely - her stepmother had transformed her into an ugly hag until the best man in England married her and gave her control over his body and goods.
She remained beautiful by day and night both, and of all the wives and paramours he had in his lifetime, Gawaine loved her best. She became mother of Guinglain and obtained Arthur's promise of mercy for her brother. Alas, she lived only five years after her wedding.
There is evidence that her name was taken from a pagan god known to Middle English writers. Both Arthur and Gawaine refer to her repeatedly as a fiend or devil.
This story is perhaps related to Celtic tales - thus Niall of the Nine Hostages, King of Tara, kissed an old crone who became beautiful and turned out to be the Sovereignty of Ireland.