Gildas mentions the war leader Ambrosius Aurelianus and Geoffrey takes his cue from him. A few words in the Historia Brittonum are the sole basis of this supposed kingship. Geoffrey keeps the reign short "dating" the whole of it in the 430's while Gildas implies that Ambrosius was active for many years.
Uncle to Arthur in Geoffrey of Monmouth. Brother to Uther Pendragon and son of King Constantine, he is raised in Brittany with his brother to avoid the scheming of the usurper Vortigern. Upon reaching manhood, the brothers return to England and overthrow Vortigern, making Aurelius king. He defeats and executes the Saxon chief, Hengist and erects Stonehenge with the aid of Merlin as a monument over the mass grave of 460 nobles massacred by Hengist. Shortly after, Ambrosius is poisoned by a son of Vortigern seeking vengeance and Uther succeeds the throne.