Iolo claimed that the original dedication of the parish church of St. Cadog in Caerleon was to St. Cadfrod, an early bishop. He was said to have been a younger son of 'King' Gadeon of Dumnonia and grandson of the great Conan Meriadoc.

Since his name means 'Battle-Brother,' the Victorian hagiographic historian, Rice Rees, tried to equate Cadfrod with another 'Brother': Adelfius, "Episcopus de civitate Colonia Londinensium," who attended the Council of Arles in AD 314. Since he was joined by a Bishop of London and there was no colonia in that town, the latter word has evidently been mistranscribed at some point. Readings of Legionensium have bolstered the Cadfrod identification, but the Colonial reference still causes a problem and the town is generally taken to be Lincoln: Lindensium.

If Cadfrod existed, he would probably have lived in the late 4th and early 5th centuries, entering the Church at a time when its influence was rapidly expanding in Britain. There were probably bishops in some twenty Roman towns by this time and there may well be some truth to the idea that there was one based at Caerleon. Cadfrod supposedly had two saintly sons, Gwrfael and Cadgyfarch.

Cadfrod only appears in the 18th century manuscripts of Iolo Morganwg. He is generally considered fictional.