An evil duke who made it a custom to kill any men who came to his court, and to imprison their ladies. He had lost an eye while fighting the Black Serpent of the Barrow.
Owain defeated him in combat and made him promise to turn his castle into a place of hospitality. Apparently, this promise was not kept because Peredur was later forced to kill the Black Oppressor to end his wicked ways.
Sources
Owain | 13th century
Peredur | 13th century