Cawrdaf is recorded as one of the sons of King Caradog Freichfras. Though the kingdom of the latter appears of have been Gwent & Ergyng, tradition ascribes that he also held sway over areas to the north, between the Wye and the Severn. When his kingdom was divided, upon his death, this region apparently fell to Cawrdaf. It was called Fferreg, though, in later centuries, it was known by the descriptive name of Rhwng Gwy ag Hafren.
He was apparently a wise monarch, one of the 'Three Chief Officers' of Britain and counsellor of King Arthur. Nothing is recorded of his reign, but Cawrdaf was a founder of churches, sometimes described as a saint with his feast day on 5th December. His endowments, at Llangowdra (Ceredigion), Abererch (Llyn) and Llangoed (Mon), were, however, not in Fferreg; and it is possible that, in old age, he abdicated in favour of his son, Caw, in order to follow his religious calling. He was supposedly a disciple of St. Illtud. Cawrdaf had several saintly children and though it is unknown how long his descendants remained Kings of Fferreg, they continued for many centuries as two of the leading patriarchal families of Brycheiniog.