In a fairly modern work, Sir Walter Scott’s Bridal of Triermain (1813), Gyneth is the daughter of Arthur by Gwendolen, who is half-fairy (or half-Djinn), the product of a brief fling.
When she came of age, she visited Arthur’s court, as Arthur had promised Guendolen to wed Gyneth to his best knight. She charmed almost all of Arthur’s knights with her wiles, and the tournament that was supposed to produce her future husband turned into a slaughter. After a young knight named Vanoc died at her feet, Merlin ended the fighting.
To punish Gyneth for her vanity, he put an enchantment on her that caused her to fall asleep for several centuries. She was awakened when Sir Roland de Vaux discovered her castle in the Valley of St. John.
See also
Djinn | The Legend of King Arthur
Source
The Bridal of Triermain | Sir Walter Scott, 1804