The Duke of Cornwall was the first husband of Igraine.
Uther Pendragon desired the Duke of Cornwall's wife, Igraine, but he was unable to defeat the Duke's forces. He sent for Merlin and asked him to change his face to resemble Cornwall's, so that he could have Igraine for just one night. The Duke of Cornwall was killed that very night by Uther's men and Igraine became Uther´s wife.
A new Duke of Cornwall took over his place and it is implied that he is going to be a rival of Arthur in the future.