According to his Legenda he was born in Cornwall and became one of many of his countrymen who moved to the continent in the wake of the Anglo-Saxon invasions and helped found the Brittonic settlement in Armorica that became established as Brittany.
The preface describes the traditional story of Vortigern, who usurps the British throne and invites Saxon warriors into the country as protection.
Arthur proceeded to win battles in Britain and in Gaul but was eventually "summoned… from human activity," paving the way for the Saxons to return.
J. S. P. Tatlock set out signs that the preface to William's Legenda followed the outlines of Geoffrey of Monmouth's Historia Regum Britanniae, written around 1136, and therefore that the date 1019 was a fiction[3] but this conclusion has been challenged by Léon Fleuriot: the preface includes material that is found in early sources but not in Geoffrey, suggesting that the author had access to some earlier document.
The text may imply that Arthur succeeds Vortigern directly whereas Geoffrey claims two kings reigned between them (Aurelius Ambrosius and Uther Pendragon).