Seisyll ap Clydog was King of Ceredigion in Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 some time in the late 7th or early 8th century. He gave his name to the later kingdom of Seisyllwg, which consisted of Ceredigion plus the region known as Ystrad Tywi

Ystrad Tywi

Ystrad Tywi is an area of south-west Wales situated on the banks of the Tywi river as it approaches the sea to join the Bristol Channel at Carmarthen...

; as such he was possibly the king responsible for the expansion. However, there is no contemporary evidence of this, and almost nothing is known of his life or reign.

History

Seisyll appears in the Harleian genealogies

Harleian genealogies

The Harleian genealogies are a collection of Old Welsh genealogies preserved in British Library, Harleian MS 3859. Part of the Harleian Collection, the manuscript, which also contains the Annales Cambriae and a version of the Historia Brittonum, has been dated to c. 1100, although a date of c.1200...

 for the kings of Ceredigion. Here, he is named as the son of Clydog or Clitauc Artgloys, and is the sixth in descent from Ceredig

Ceredig

Ceredig ap Cunedda, , king of Ceredigion, may have been born c. 420 AD in the Brythonic kingdom of Manaw Gododdin , centred on the Firth of Forth in the area known as Yr Hen Ogledd.Little is known of him...

, the traditional founder of Ceredigion. Harleian also names him as the father of Arthgen, presumably the Arthgen, King of Ceredigion whom the Annales Cambriae

Annales Cambriae

Annales Cambriae, or The Annals of Wales, is the name given to a complex of Cambro-Latin chronicles deriving ultimately from a text compiled from diverse sources at St David's in Dyfed, Wales, not later than the 10th century...

record as dying in 807. Seisyll's name appears to derive from the notional Late Latin

Late Latin

Late Latin is the scholarly name for the written Latin of Late Antiquity. The English dictionary definition of Late Latin dates this period from the 3rd to the 6th centuries AD extending in Spain to the 7th. This somewhat ambiguously defined period fits between Classical Latin and Medieval Latin...

 *Saxillus, which comes from the same root as the Welsh sais, meaning Saxon

Saxons

The Saxons were a confederation of Germanic tribes originating on the North German plain. The Saxons earliest known area of settlement is Northern Albingia, an area approximately that of modern Holstein...

 or Englishman

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

.

Seisyll ap Clydog is generally taken to be the Seisyll for whom Seisyllwg was named. As such, some historians such as John Edward Lloyd

John Edward Lloyd

Sir John Edward Lloyd , was a Welsh historian, the author of the first serious history of the country's formative years, A History of Wales from the Earliest Times to the Edwardian Conquest, 2 vols...

and Egerton Phillimore believed that he was the king who founded that kingdom. However, contemporary sources continue to use the name Ceredigion for the realm of Seisyll and his successors. As "Seisyllwg" may represent a substantially later political development.