It is assumed to have been named for Seisyll, King of Ceredigion in the 7th or early 8th century, and as such he is traditionally regarded as its founder.
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It is evidently named after Seisyll, king of Ceridigion in the 7th or early 8th century, but it is unknown if he was directly responsible for its establishment.
Hywel Dda unites the kingdoms of Dyfed and Seisyllwg to create Deheubarth.