That is she who is in my castle, the very old lady,Who is actually your aunt, Arthur’s half-sister,The duchess of Tintagel’s daughter, whom noble UtherAfterwards begot Arthur upon, who now is king.(2463 - 2466)
The allusion to the legend of Arthur’s beginnings in this passage refers to the story of how Uther, in love with a beautiful married woman named Igraine, asked Merlin to change him into her husband for a night so he could sleep with her. On that night, the story goes, they conceived Arthur. Morgan seems to have taken a leaf out of Merlin’s book when it comes to her magic of choice, seeing as how she’s recently shape-shifted Bertilak into the Green Knight.