Wikipedia
Tewdrig was a king of the post-Roman Kingdom of Glywysing. He abdicated in favour of his son Meurig (Maurice) and retired to live a hermitical life, but was recalled to lead his son's army against an intruding Saxon force. He won the battle, but was mortally wounded.
The context of the battle is one of Britons versus invading Saxons, without explicit religious overtones. Since Tewdrig held to a religious lifestyle and was killed while defending a Christian kingdom against pagans, by the standards of that day Tewdrig is considered to be a martyr and a saint. The Latin form of his name is given as 'Theodoric' and his feast day is 1 April. Tewdrig's name appears in a genealogy of Jesus College MS 20, in the line of one of his descendants, 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. The ancient histories of the kingdoms of Gwent and Glywysing are intertwined, and he may have ruled both kingdoms.