Dictionary of National Biography, 1885-1900/Llywelyn ab Seisyll
LLYWELYN ab SEISYLL or SEISYLLT (d. 1023?), king of Gwynedd, was a Welsh chieftain, not of the royal line, who married, if the tradition of a later time can be trusted, Angharad, daughter of Maredudd. son of Owain, son of Howel Dda [q. v.] (Gwentian Brut, s.a. 994),and thus became associated with the greatest house in South Wales. Llywelyn lived in a time of exceptional confusion. In North Wales the stock of the royal house of Gwynedd had been replaced. on the throne by a vigorous usurper, Aeddan ab Blegywryd. The inroads of the Danes and the advances of the English power were fatal to settled rule in North and South Wales alike. Llywelyn managed, however, to slay Aeddan and his four sons. This event probably happened in 1017, or possibly 1018, the year after the accession of Cnut in England (Brut y Tywysogion, p. 35; Annales Cambriæ,p. 22). Llywelyn now took possession of the throne of North Wales, thus bringing in the family of Howel Dda in the person of his descendants, and representing some sort of triumph of South Welsh over North Welsh. Llywelyn'a brief reign was one of exceptional prosperity. He is styled 'supreme king of Gwynedd, and the chief and most renowned king of all the Britons' (Brut y Tywysogion, p. 37). 'In his time,' wrote the Welsh chronicler (ib. p. 37), 'it was usual of the elders of the kingdom to say that his dominion was, from one sea to the other, complete in abundance of wealth and inhabitants, so that it was supposed that there was neither poor nor destitute in all his territories, nor an empty hamlet, nor any deficiency.' This indicates that under Llywelyn that restoration of the North Welsh power began which attained its highest point in the reign of his more famous son Gruffydd ab Llywelyn (d. 1063) [q. v.] But in 1020 or 1022 Llywelyn had to face a formidable enemy. An Irish impostor named Rein claimed to be the son of Maredudd ab Owain, Llywelyn's father-in-law, formerly king of South Wales. Rein was so successful as to obtain general recognition throughout Deheubarth (South Wales). Llywelyn was still sufficiently connected with southern affairs to fear the growth of his power. He accordingly marched with an army into South Wales. Rein, 'after the manner of the Irish,' 'proudly and ostentatiously' exhorted his men to fight, with many boasts of victory. After a sharp struggle the men of Gwynedd prevailed, and Rein fled 'shamefully, like a fox.' The battle was fought at Abergwili, near Carmarthen. Rein was heard of no more, and perhaps perished in the battle (Annales Cambriæ, p. 23). Llywelyn, by cruelly devastating the south, vindicated his position as chief king of the Welsh. Next year he died. The date is either 1021 or 1023, probably the later year. He left a brother named Cynan, who was slain four years later. His son Gruffydd ab Llywelyn (d. 1063) [q. v.] was for a time driven from Gwynedd by a restoration of the rightful line. The Gwentian chronicler celebrates Llywelyn's virtues in war and peace, and couples him with his son as 'the noblest princes that had been until their time in Wales.'
[Annales Cambriæ, Brut y Tywysogion, both in Rolls Ser.; Brut y Tywysogion, ed. Rhys and J.G. Evans; Gwentian Brut y Tywysogion, Cambrian Archæological Association.]