[2] Tewdrig's name appears in a genealogy of Jesus College MS 20, in the line of one of his descendants,[3] but the only substantive information about the person comes from the twelfth century Book of Llandaff.

The Book of Llandaff places Tewdrig's story in the territory of the historical Kingdom of Gwent (the southeastern part of modern Monmouthshire), though it states that he was a king of Glywysing.

At some point in his reign, he abdicated in favour of his son Meurig in order to live a hermitical life at Tintern, a rocky place near a ford across the River Wye.

He asked to be taken to Ynys Echni (called Flat Holm in English) for burial, but got no further than Mathern on an inlet of the Severn estuary, where he languished briefly and died.

On returning to secular service due to military necessity, Tewdrig is given the prophecy that he will be successful but will be mortally wounded; that a vehicle pulled by two stags, yoked, will appear and carry him towards his destination of Ynys Echni, but that he will die in peace three days after the battle.

In 1958 the writer and illustrator Fred Hando recorded a story told to him by an old woman, long resident in Mathern, who claimed to have seen, in 1881, the opened coffin containing Tewdrig's body, with a wound to his skull still visible.

[10] Turner's History of the Anglo-Saxons (1799) repeats the accounts of the Book of Llandaff and Bishop Godwin (citing Ussher as the source), but then adds that the Saxons in question were those of Wessex, led by Ceolwulf.