The Gwentian Chronicle records "Llywelyn son of Seisyllt lord of Maes Essyllt…although but a youth not more than fourteen years of age" marrying in 994.
He succeeded in 999 as King of Gwynedd and Dyfed. The Gwentian Chronicle records that "Llewelyn son of Seisyllt" killed in battle "Aeddan son of Blegywyrd and his four nephews" in 1015 and "took the government upon himself". The Chronicle of the Princes of Wales records that "a certain Scot falsely pretended to be the son of king Maredudd and caused himself to be named king…Rein" in 1020 and "Llywelyn son of Seisyll, supreme king of Gwynedd and the chief and most renowned king of all the Britons made war against him".
The Annales Cambriæ record the death in 1023 of "Lewelin filius Seisil". The Annals of Tigernach record that “Leobelín rí Bretan” died in [1021/23]. The Chronicle of the Princes of Wales records that "Llywelyn son of Seisyll died" in 1021. The Gwentian Chronicle records that "Llywelyn son of Seisyllt and Cynan son of Seisyllt" opposed "the Scots [who] came to Carmarthen accompanied by Hywel and Maredydd sons of Edwin son of Einion" in 1021 but that Llywelyn was killed in the battle. 1
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Nothing is known of his father, but his mother, Prawst, was, according to late pedigrees, the daughter of Elisedd, a younger son of Anarawd ap Rhodri Mawr. Since he himself m. Angharad , daughter of Maredudd ab Owain ap Hywel Dda, he had distant claims to succession in both Deheubarth and Gwynedd, which in the circumstances of the time could be translated into reality by a leader of force and ambition. Such a man was Llywelyn, and by his defeat of the usurper, Aeddan ap Blegywryd, in 1018, and of Rhain, an Irish pretender, in 1022, he became master of the south. He also extended his rule for a few years over a large part of Wales, a memorable period in Welsh annals judging from entries in the chronicle. His chief claim to fame, however, is the fact that he was the father of Gruffudd ap Llywelyn, whose notable career was made possible by the example of the father, and whose claim to power was largely based on the short ‘de facto’ overlordship of Llywelyn. 2
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