Ragnelle was born into a noble family, the sister of Sir Gromer Somer Joure, a knight whose lands King Arthur had given to Sir Gawain. The poem that tells her story, The Wedding of Sir Gawain and Dame Ragnelle, was written down around 1450 in a manuscript now held at the Bodleian Library, though the tale itself is far older, with roots in Celtic sovereignty myths where an ugly hag transforms into a beautiful maiden when the rightful king proves worthy. In her youth, Ragnelle was beautiful, but her stepmother - or in some tellings her brother Gromer himself - transformed her by enchantment into the loathly lady, a creature of such grotesque appearance that no man could look upon her without revulsion. The curse could only be broken if the best and most courteous knight in England married her of his own free will and granted her sovereignty over his body and goods.

The defining encounter of Ragnelle's life came in Inglewood Forest, where King Arthur had gone hunting alone. After killing a great hart, the king was confronted by Sir Gromer Somer Joure, the knight whose lands he had given away. Gromer was armed and furious, and he threatened to kill the unarmed king on the spot. When Arthur pleaded for his life, Gromer agreed to spare him on one condition: Arthur must return in exactly one year and one day with the answer to the question "What is it that women desire most?" If the king brought the wrong answer, he would lose his head. Arthur swore the oath and rode back to Carlisle in heavy sorrow, telling no one what had happened except his nephew Gawain.

For a year Arthur and Gawain searched for the answer, questioning every woman they met - queens, peasants, nuns, and witches - and receiving a different reply from each. Some said women desired beauty, others riches, others to be widowed and wed at will, others to be thought young and fresh. None of the answers matched, and as the appointed day drew near, Arthur rode alone into Inglewood Forest to keep his oath, despairing that he would ever find the truth. There he met Dame Ragnelle, riding a beautiful palfrey and carrying a lute, but herself so ugly that the king could barely look at her. She was broad-fronted, with a nose like a boar's snout, great tusks, and a neck covered in sores. She told Arthur that she knew the answer he sought, but she would tell him only on one condition: that Sir Gawain, the most courteous knight in the land, must marry her. Arthur refused at first, unwilling to sacrifice his nephew, but Ragnelle warned him that without her help he was a dead man. The king rode back to Carlisle and told Gawain everything, and Gawain, without hesitation, agreed to marry the loathly lady to save his king's life.

The wedding was one of the most extraordinary in Arthurian legend. When Arthur offered to hold the ceremony in secret, early in the morning with no guests, Ragnelle refused. She insisted on a full public wedding at mid-day, with a feast for all the court, and she wore a bridal gown worth three thousand gold pieces that only made her grotesque appearance more startling. At the feast she ate with voracious appetite, consuming as much as six other guests combined, and she made no effort to hide her ugliness or her hunger. The court was appalled, and even Gawain, who had agreed to the marriage, must have wondered what he had promised. But he kept his word with perfect courtesy, treating his bride with the same respect he would show the fairest lady in the land.

The transformation came in the bridal chamber. When Gawain turned to kiss his wife, he found not the hag he had married but the most beautiful woman he had ever seen. Ragnelle explained that her stepmother's enchantment had trapped her in the loathly form, and that only a husband's grant of sovereignty could break it completely. But there was a catch: Gawain must choose whether to have her beautiful by night and ugly by day, or beautiful by day and ugly by night. He could not have both. Gawain thought long and hard, and then he did the one thing that broke every version of the curse in every telling of the tale: he gave the choice to her. "Do as you list," he said, putting the decision in her hands. At those words, the enchantment shattered entirely. Ragnelle was freed to be beautiful both day and night, and she told Gawain that of all the wives and lovers he would ever have, she would be the one he loved best. They lived in perfect harmony for five years, and she bore him a son named Gyngolyn. But the poem ends with sorrow as well as joy: Dame Ragnelle died after only five years of marriage, and Gawain grieved for her all the rest of his life.