A daughter of the goddess Don and god Beli, and wife of Llyr in Welsh mythology, mother of Bran the Blessed, Branwen, Manawyddan, and Efniessin, and Niessin by Eurowysedd. As Eurowysedd is thought to be "Ostorius," a Roman general, this may be a later interpolation. Or, conversly, as the family of Llyr are the keepers of the Grail/Cauldron of Rebirth, she may have been raped, like the Grail maidens in The Elucidation, a prologue attatched to Perceval.
As her name means sovereignty, it is no stretch to assume that she is a goddess of the land, a deified representation of the Isle of Prydein (Britain), much like the goddess Eriu represents Ireland in their mythology. Marriage to Penardun would ensure kingship of the land, and her sons and daughter would also hold that kingship. Unfortunately, her brother Casswallan usurped her sons Bran the Blessed, his son Caradawg ap Bran, and Manawyddan--all the rightful heirs to the throne of Britain.