Uther Pendragon

 was the father of Arthur Pendragon.

Uther fancied Igerna, the wife of Gorlois. While Gorlois was away fighting a battle, a wizard called Merlin disguised Uther to look like Gorlois. He impregnated Igerna, who believed it was her husband. After it was learned that her husband had died during the battle, Uther married Igerna. (PROSE: One Fateful Knight)