Morgause

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Morgause was the half-sister of Arthur who married Lot. She was the mother of Gawain, Gaheris, Agravain, Gareth, and Mordred. According to the Enfaces Gawain, Lot was her page with whom she had an intrigue, as a result of which Gawain was born. In Malory she is Lot's queen who, as the result of an amatory encounter with Arthur (who did not know they were related), gave birth to Mordred. Morgause was slain by her son Gaheris who found her in a relationship with Lamorak, whose father, Pellinore, had slain Lot. 

Morgause does not seem to have been the original name of this character. In Geoffrey, the wife of Lot is called Anna, sister to Arthur. In De Ortu Waluuanii the part taken by Morgause in theEnfaces Gauvain is assigned to Anna; and the name Morgause itself seems to be in origin a territorial designation rather than a personal name, for in Diu Crone Gawain's mother is called Orcades or Morchades, which seems to be taken from the Orkneys (in Latin: Orcades), the name of one of Lot's kingdoms, and Morchades seems to be a variant form of Morgause.