Answer: Uther Pendragon and Igraine
Legend has it that Merlin magically gave Uther Pendragon the exact likeness of his rival, Gorlois of Tintagel, Duke of Cornwall. This was so that he could father a child with Gorlois's wife, Igraine, with whom he was besotted.
In his (not entirely reliable) History of the Kings of Britain, 12th-century writer Geoffrey of Monmouth named Tintagel as the place where Arthur was conceived.
Image: A painting of Uther Pendragon