Morgause

Morgause is sent to Arthur's court as a spy for her husband King Lot, and stays a month, and neither she nor Arthur knows they share a mother. Malory is very brief about it. The child is Mordred.

What she also produces is the house of Orkney: Gawain, Agravaine, Gaheris and Gareth, four brothers whose loyalty to each other is the strongest bond in the book and the most destructive. They murder Lamorak because his father killed theirs. Agravaine engineers the ambush of Lancelot. Gawain's grief for Gareth drives the final war.

She is murdered by her own son Gaheris, who finds her in bed with Lamorak and beheads her — and spares Lamorak, because killing an unarmed man would be dishonourable. Malory reports this without comment, which is somehow worse.

What the tradition does with her is worth watching, because it changes the crime. In Geoffrey, Arthur's sister is called Anna and there is no incest at all; Mordred is simply a nephew who usurps. The French cycle introduces the half-sister and the unwitting conception, and in doing so converts a political betrayal into something that looks like inherited guilt — a kingdom brought down by a sin its king committed without knowing he was committing it.

Malory keeps that and adds the household. Morgause's four sons are the most cohesive bloc at the Round Table and the most destructive: their vendetta against Pellinore's line produces the murder of Lamorak, Agravaine forces the exposure of the affair, and Gawain's grief drives the last war. Every strand of the ending runs back through her, and the book gives her barely a page of her own.