He is also known in Welsh tradition as Gwrtheyrn, meaning "supreme leader." He is said to have been ruler of Britain in the fifth century and to have invited Jutish mercenaries under Hengist and Horsa into the island of Britain. Hengist and Horsa then turned on him and began to carve out kingdoms for themselves. This was the first appearance of the ancestors of the English in Britain. Because of this, he is regarded as the archtraitor in Celtic tradition.