Merlin & Uther
Merlin put a spell on Uther, so he could lay with Ygraine while she was sure she was with her man
I said slowly, holding him with my eyes: "Listen now, and I will tell you who the horsemen were. Last night, in the dark, King Uther Pendragon rode this way, with myself and two others. He rode to Tintagel, but he did not go by the gate-house and the bridge. He rode down the valley, to the shore, and then climbed the secret path up the rock and entered the castle by the postern gate. Why do you shake your head? Don't you believe me?"
"Lord, everyone knows the King had quarrelled with the Duke. No one could get in, least of all the King. Even if he did find the postern door, there's none would dare open it to him."
"They opened last night. It was the Duchess Ygraine herself who received the King into Tintagel."
"Wait," I said. "I will tell you how it happened. The King had been changed by magic arts into a likeness of the Duke, and his companions into likenesses of the Duke's friends. The people who let them into the castle thought they were admitting Duke Gorlois himself, with Brithael and Jordan."
Under its dirt the boy's face was pale. I knew that for him, as for most of the people of this wild and haunted country, my talk of magic and enchantment would come as easily as stories of the loves of kings and violence in high places.
He said, stammering: "The King - the King was in the castle last night with the Duchess?"
"Yes. And the child that will be born will be the King's child".
(From: "The Hollow Hills" by Mary Stewart)
get back move on