Peibio was a younger son of King Erb of Gwent & Glywysing. Upon the latter's death around the early AD 570s, his vast kingdom, covering most of South Wales, was divided amongst his sons. Peibio took the lesser eastern portion known as Ergyng and may have been quite young upon his accession. This stretched from the River Monnow eastwards, covering the Wye Valley and most of what is now Herefordshire (England). Peibio occurs in three charters in the Book of Llandaff which Dr Wendy Davies takes to be genuine. She dates them to AD 575, 80 & 85.

This unfortunate king apparently suffered from leprosy. Hence his epithet of Clafrog - the Leprous - though, in subsequent centuries, this was mistaken for Glafoeriog - the Dribbler. Despite this affliction, Peibio married the daughter of one King Constantine (probably St. Constantine, King of Dumnonia) and fathered a family of at least four sons (Cynfyn, Gwyddgi, Cynwst & Tewdr) and a single daughter. Details of his life are mostly known via the exploits of the latter, Princess Efrddyl.

Upon returning from a victorious military campaign - against his quarrelsome British neighbours or encroaching Mercians, we are not told - King Peibio was horrified to discover that his unmarried daughter was pregnant. Although the story that he was the father of her child appears to have been nothing but malicious gossip, shame and disgust led him to have the poor girl executed for adultery. She was tied up in a sack and cast into the River Wye but, miraculously, she always returned safely to the bank. So Peibio had her burnt on a funerary pyre at Madley, in Herefordshire. However, when he returned, next day, to recover her body, the King found Efrddyl sitting upon a huge stone, amongst the ashes, clutching her new-born baby boy.

Upon seeing Peibio, his little grandson reached out to the King and kissed him on the cheek. The scales of his leprosy immediately fell away and King Peibio found himself cured the disease. His heart melted and he welcomed them both back to the Royal Court. When the boy - Dyfrig by name - grew up, Peibio gave him the whole area around Madley called Ynys Efrddyl, where he became a great evangelist of Christianity.

Peibio may be the same as King Propius who was healed of his blindness by St. Dewi, though this is far from certain. Confused legends indicate that, in later years, Peibio's armies fought with those of his brother, King Nynnio of Gwent, over grazing rights in their two kingdoms. While the civil war was raging, both areas were overrun by a Penllyn warlord named Rhita Gawr (the Giant). Peibio was presumably killed in the fighting but, with Rhita's subsequent defeat on Mount Eryri (Snowdon), the former's son, Cynfyn was able to inherit the Crown of Ergyng.