Ambrosius Aurelianus
The "Historia Brittonum" provides the earliest mention of Gwrtheyrnion. According to the text, Vortigern fled to Gwrtheyrnion after Saint Germanus of Auxerre had castigated him for his various sins. He eventually died, and his son Pascent received Gwrtheyrnion and Buellt from Ambrosius Aurelianus.