By 1088 Bernard de Neufmarché mentioned 'all the tithes of his lordship which he had in Brycheiniog in the woods and plains' as well as Glasbury.
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In the 7th century, the inheritance of a woman, Ceindrych, brought the kingdom into the hands of Cloten of Dyfed and Brycheiniog.
However, it was an attack on Brycheiniog by the Marcher Lords Humphrey de Bohun and Roger Mortimer in 1276 which led to the final breakdown of the peace between England and Wales after which Llywelyn's domain was reduced to just his lands in Gwynedd.
Saint Gwladys ferch Brychan or St Gladys (Latin-Claudia), daughter of King Brychan of Brycheiniog, was the queen of the saint-king Gwynllyw Milwr and the mother of Cadoc "the Wise", whose vita may be the earliest saint's life to mention Arthur.
Rhain ap Cadwgan or Cloten (fl. 8th century), King of Dyfed and Brycheiniog
Cloten had married the princess of Brycheiniog and briefly united the two kingdoms.
The Welsh Bruts state that "Rhys ap Tewdwr, king of Deheubarth, was slain by the Frenchmen who were inhabiting Brycheiniog."
The church is dedicated to Saint Mabyn or Mabena, who was regarded in local tradition as one of the many children of Brychan, a Welsh saint and King of Brycheiniog in the 5th century.