Seisyll ap Clydog was the 4x great grandson of King Ceredig Ceredigion's eldest son, King Usai. He was an early 8th century King of Ceredigion who invaded Dyfed and managed to conquer Ystrad Tywi. His enlarged kingdom became known as Seisyllwg in his honour. He was the great great grandfather of King Gwrgan ap Meurig.
Records of King Seisyll date back to the 11th century. He is generally considered historic.
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