The ruler of Dyfed in the Welsh tale that bears his name.
He was the father of Pryderi. He became, for a time, the ruler of Annwn, having exchanged kingdoms with Arrawn.
He is non-Arthurian, though some scholars view him as the origin of Pelles. The Spoils of Annwn makes reference to his story.
Source
Preiddeu Annwfn | Attributed to Taliesin, c. 900