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Arthurian Mirror/Role Reversal AU, where Morgan is the Once and Future Queen, leader of the Dames of the Round Table, who search for the Holy Clau or Holy Rood.
Meanwhile Arthur becomes the Supernatural Raider/Wild Man called "Arthur la Guivre", who terrorizes Morgan's kingdom but ultimately rescues his sister after the final battle, bringing her over to his Otherworldly realm under a Mountain.
Sebile becomes the Sir Kay of this AU.