Faustus, in the Historia Brittonum, is a son of Vortigern whom he had fathered on his own daughter, who he had taken as a wife.
Faustus and Germanus[]
From the Historia Brittonum, as translated by Giles:
39. In the meantime, Vortigern, as if desirous of adding to the evils he had already occasioned, married his own daughter, by whom he had a son. When this was made known to St. Germanus, he came, with all the British clergy, to reprove him: and whilst a numerous assembly of the ecclesiasties and laity were in consultation, the weak king ordered his daughter to appear before the, and in the presence of all to present her son to St. Germanus, and declare that he was the father of the child. The immodest woman obeyed; and St. Germanus, taking the childc said, “I will be a father to you, my son; nor will I dismiss you till a razor, scissors, and comb, are given to me, and it is allowed you to give them to your carnal father.” The child obeyed St. Germanus, and going to his father Vortigern, said to him, “Thou are my father; shave and cut the hair of my head.” The king blushed, and was silent; and, without replying to the child, arose in great anger, and fled from the presence of St. Germanus, execrated and condemned by the whole synod.
In section 48:
The fourth [son of Vortigern] was Faustus, born of an incestuous marriage with his daughter, who was brought up and educated by St. Germanus. He built a large monastery on the banks of the river Renis, called after his name, and which remains to the present period.
Faustus of Riez[]
Faustus of Riez was born in Britain between 405 and 410. Influenced by his mother, famed for her sanctity, he abandoned secular life when a young man and entered the monastery of Lérins and was soon ordained to the priesthood and was chosen in 432 to be head of the monastery, in place of Maximus who had become Bishop of Riez.
After the death of Maximus, Faustus was made Bishop of Riez. He was zealous opponent of Arianism and so was banished by the Gothic king Euric, but returned to his position of bishop after Euric’s death. He died some time about 490–495.
It is uncertain whether this Faustus is the same person as Vortigern’s son or whether two separate people have been confused with one another. The “monastery on the banks of the river Renis” might well be a garbling of Faustus being a monk who was later made Bishop of Riez (Regiensis), but no monastery, so far as is known, was “called after his name”.