Dictionary of National Biography, 1885-1900/Ambrosius Aurelianus
AMBROSIUS AURELIANUS (fl. 440), called Emrys by Welsh writers, was a leader of the Britons in the fifth century, about whose history little that is certain can be extracted from the mass of legend that has gathered about his name. Our earliest authority, Gildas, speaks of him with enthusiasm, but with little definiteness, as the leader who checked the victorious advance of the Saxon invaders. He describes Ambrosius as ‘courteous, faithful, valiant, and true; a man of Roman birth who had alone survived the conflict, his kindred, who had worn the purple, having perished in the struggle; his descendants, greatly degenerated in these days from the excellence of their ancestors, still provoke their conquerors to battle, and by the grace of God their prayers for victory are heard.’ Geoffrey of Monmouth and other later writers represent Ambrosius as the son of Constantine, who was elected emperor in Britain, Gaul, and Spain during the reign of the Emperor Honorius; but their accounts of Constantine's life and death are so utterly irreconcilable with known facts that no reliance can be placed on their statements. Geoffrey's account is shortly as follows: Aurelius, Ambrosius, and Uther Pendragon were the sons, by a Roman wife, of Constantine the Tyrant, who was murdered by Vortigern after a reign of ten years. On their father's death Constantine's two sons fled to the king of Armorica, but returned after some years, during which Vortigern, as king of Britain, had been forced to rely upon the aid of the Jutes, under Hengist and Horsa, for protection against the Picts. On his return Ambrosius was anointed king, and proceeded to attack Vortigern, whom he defeated and killed at Genoreu. Ambrosius was then opposed by Hengist, whom he defeated with great slaughter at Maisbeli. Hengist was soon afterwards put to death. Ambrosius reigned as king for some years longer, and was poisoned at Winchester, where he lay sick, by a Saxon named Copa, disguised as a physician.
What may be looked upon as ascertained is that Ambrosius was of Roman origin, and probably descended from Constantine; that his birth attached to him Romans or Romanised Britons; and that he was a successful opponent of the advance of the Saxon invaders, whom he drove back and confined within the limits of the isle of Thanet. The Welsh writers give Ambrosius the title of ‘Gwledig,’ applied by them to those who occupied the place of Dux Britanniæ and Comes Litoris Saxonici; it is the epithet given to Maximus and Constantine, who had both borne the title of emperor.
[Gildas, De Excidio Britanniæ; Geoffrey of Monmouth, Historia Britonum.]