One of Igraine's daughters married to Lot. Arthur first meets her when she visits him in Carlion with her sons Gawain, Gaheris, Agravine, and Gareth. The official reason for her is to deliver a message from Lot, but her main purpose was to spy and gather information. She was still a very beautiful woman and Arthur desired her. Not knowing that she was his half-sister, he slept with her and got her pregnant with Mordred. Malory at that point leaves the reader unclear as to whether Morgawse knew of the familial relationship.
more to come...
