Seisyllwg
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Seisyllwg was a kingdom of medieval South Wales.
[edit] Origins
Seisyll (born c. 665) was a direct descendant of King Ceredig's eldest son, Usai. He was a late 7th century king of Ceredigion who invaded Dyfed and managed to conquer Ystrad Tywi. His enlarged kingdom of Ceredigion and Ystrad Tywi became known as Seisyllwg in his honour.
[edit] Merger with Dyfed
In the tenth century Hywel Dda united both Seisyllwg and Dyfed under his own rule, and the new kingdom became known as Deheubarth.