In the The Vulgate Cycle (1215 to 1235) Morgan Le Fay is married to Uriens, and is Queen Guinevere's lady in waiting. She fell fell in love with the King's nephew, Giomar, but Guinevere stops that romance. Morgan is piqued into betraying Guinevere's affair with Lancelot to King Arthur. She too became attracted to Sir Lancelot though he rejects her. She imprisons Lancelot in order to try to gain his love, but she never succeeds. The Cistercian monks who wrote the stories of the Vulgate Cycle certainly did not like Morgan le Fay.
One story recounts that Morgan Le Fay cast a spell over the sleeping Lancelot and then held him prisoner in the dungeon of her castle. She and three other fairy queens would not free him until he choose one of them as his mistress. Lancelot refused them, and admitted his love for Queen Guinevere.
She sent the Green Knight to Camelot in order to frighten Guinevere to death. e.
Sir Lancelot, knight of the Round Table