Llywelyn ap Seisyll

Llywelyn ap Seissyll (died 1023) was an 11th-century King of Gwynedd, Powys and Deheubarth.
Llywelyn was the son of Seisyll, a man of whom little is known. Llewellyn first appears on record in 1018, that year he defeated and killed Aeddan ap Blegywryd along with four of his sons and obtained Gwynedd and Powys.
In 1022, a man named Rhain the Irishman was made king of Deheubarth, he claimed to be a son of Maredudd ab Owain, whose daughter Angharad had married Llywelyn. Llywelyn made war against Rhain, they fought a battle at Abergwili, and after a “slaughter on both sides” Rhain was killed allowing Llywelyn take control of Deheubarth.
Llywelyn, after his success against Rhain, died in 1023. The Brut y Tywysogion portrays Llywelyn’s reign as one of prosperity saying “complete in abundance of wealth and inhabitants; so that it was supposed there was neither poor nor destitute in all his territories, nor an empty hamlet, nor any deficiency.” Llywelyn was called "King of the Britons" by the Annals of Ulster.
Llywelyn had one son called Gruffydd; he did not succeed his father, possibly because he was too young to do so. Gruffydd went on to become the first King of Wales, however he was killed by his own men in 1063. Gruffydd’s own sons Maredudd and Idwal died in 1069, fighting at the Battle of Mechain.