Morgan le Fay

Morgan le Fay is the sister of King Arthur and mother of Gawain, Agravain, Gaheris, Gareth and Mordred. Morgan was the daughter of Tintagel, the fae child conceived by Igraine, the progeny of the British Isles... and later, Artoria's stepsister, as Morgan's mother Igraine wed King Uther.

In Fate/Apocrypha:

Kairi Sisigou has dreams of Mordred's past showing how Morgan encouraged her to join the Knights of the Round Table, and later revealed to Mordred that she was King Arthur's son and encouraged her to turn against Arthur.

In Fate/Grand Order Cosmos in the Lostbelt:

She established an authoritarian monarchy in the Lostbelt Britain and has made the fairies suffer under her oppressive rule for 2000 years. As the highest-ranking fairy, she is a genius sorcerer who has mastered Rhongomyniad, the Lance that Shines to the Ends of the World, as a form of Magecraft.