Истрад-Тиви
At the start of the 8th century Ystrad Tywi was part of the kingdom of Dyfed. Around the year 730 Seisyll ap Clydog, king of Ceredigion, captured Ystrad Tywi from Rhain ap Cadwgan, king of Dyfed, and annexed it to his own kingdom. The name Seisyllwg was given to the new enlarged kingdom. As a result, Brycheiniog was once again detached from Dyfed; Ystrad Tywi had previously joined the two.