AMBROSIUS, Aubelianus, a leader of the Britons during the 5th century. He is said, on somewhat doubtful authority, to have been a son of Constantine, who was elected emperor by the Roman army in Britain in 407. The usually received account of his life, based chiefly upon the history of Geoffrey of Monmouth, contains much that is evidently fabulous. It seems probable that he was educated at the court of Aldroën, king of Armorica, who sent him over with a strong force to assist his countrymen against the Saxons, whom Vortigern had invited to Britain. There is also little doubt that, having defeated Vortigern, he was chosen to succeed him as king of Britain. Geoffrey also states that he built Stonehenge (see Stonehenge), that he defeated Hengist, and that he compelled the Saxons to surrender at York; but these stories are inherently improbable. The circumstances of his death are involved in equal obscurity. According to Geoffrey’s account, he died of poison at Winchester; but others state that he was killed in a battle with Cerdic the Saxon in 508.