The diocese was reputedly founded in 560 or earlier by Saint Teilo, during the monastic movement initiated by Saint Dubricius who presided over several monasteries in Ergyng, including Hentland and Moccas.
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Dubricius was only active in Ergyng and Gwent, while Teilo's associations with Llandaff have been transferred from his great abbey at Llandeilo Fawr.
Meurig ap Tewdrig, King of Gwent and the husband of Onbrawst, daughter of Gwrgan Fawr, who was a cousin of Dubricius