He married Matilda de Braose (died 1248) who betrayed the dynasty's chief castle of Carreg Cennen to the Anglo-Normans in 1248, against the interests of her son Rhys.
A Welsh chronicle, the Brut y Tywysogyon, records under the year 1248: "Rhys Fychan ap Rhys Mechyll regained the castle of Carreg Cennen, which his mother had treacherously placed in the power of the French, out of enmity for her son.
"[2] He had four sons[3][1][4] and a daughter, Gwenllian, who married Gilbert Talbot (died 1274), the grandfather of Gilbert Talbot, 1st Baron Talbot (died 1345/46),[5] who claimed the ancient armorials of the House of Deheubarth, assumed as arms of alliance to a great princess in place of his own paternal arms.
[6] The assumption about Gwenllian, however, was unfounded [clarification needed] as Rhys Mechyll, Lord of Dinefwr, also had male heirs[7] who acceded to the arms of the House of Deheubarth.
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