Morgause

Morgause is Arthur's half-sister and the wife of King Lot. Her sons are Gawain, Agravaine, Gaheris and Gareth, by Lot and Mordred by Arthur. In Malory, she and Arthur have an amorous encounter and engendered Mordred; Arthur remains relatively blameless since he did not know they are related, but we are not as certain whether she knows the encounter is incestuous or not. She is killed by Gaheris -her own son-while in bed with her lover Lamorak. Lamorak's father, Pellinore, had killed King Lot. Morgause is sometimes renamed, eliminated or conflated with Morgan le Fay, as in John Boorman's movie Excalibur.

Bibliography
The Illustrated Encyclopedia of Arthurian Legends. Ed. Ronan Coghlan. New York: Barnes and Noble, 1993.
New Arthurian Encyclopedia. Ed. Norris J. Lacy. New York: Garland, 1996.