Ambrosius Aurelianus is one of the few people Gildas identifies by name in his sermon. Following the destructive assault of the Saxons, the survivors gather together under the leadership of Ambrosius, who is described as "a gentleman who, perhaps alone of the Romans, had survived the shock of this notable storm. Certainly his parents, who had worn the purple, were slain in it. His descendants in our day have become greatly inferior to their grandfather's excellence." According to Gildas, it is Ambrosius who organizes the survivors into an armed force, and achieve their first military victory over the Saxon invaders.
Two points in this brief description have attracted much scholarly commentary. The first is what Gildas meant by saying Ambrosius' parents "had worn the purple": does this mean that Ambrosius was related to one of the Roman Emperors, perhaps The House of Theodosius[?] or a usurper like Constantine III? Perhaps Gildas' statement inspired -- or is explained by -- the Welsh tradition that he was one of the sons of the emperor Magnus Maximus and his Celtic wife, Helen. The second question is the meaning of the word avita: does it mean "ancestors", or did Gildas intend it to mean more specifically "grandfathers" -- thus indicating Ambrosius lived about a generation before the Battle of Mons Badonicus? The lack of information for this period prevents us from decisively answerering these questions.
The Historia Brittonum preserves several snippets of lore about Ambrosius. The most significant of these is the story about Ambrosius, Vortigern, and the two dragons beneath Dinas Emrys in Chapters 40-42. This story was later retold with more detail by Geoffrey of Monmouth in his Historia Regum Britanniae[?], conflating the personage of Ambrosius with the Welsh tradition of Merlin the visonary, known for oracular utterances that foretold the coming victories of the native Celtic inhabitants of Britain over the Saxons and the Normans.
But there are smaller snippets of tradition preserved in Historia Brittonum: in Chapter 31, we are told that Vortigern ruled in fear of Ambrosius; later, in Chapter 66, various events are dated from a battle of Guoloph, which is said to have been between Ambrosius and Vitolinus; lastly, in Chapter 48, it is said that Pascent, the son of Vortigern, was granted rule over the kingdoms of Buellt[?] and Gwrtheyrion[?]. It is not clear how these various traditions relate to each other, but we should not assume without careful study that they all come from the same tradition.
Because Ambrosius and Vortigern are shown in the Historia Brittonum as being in conflict, some historians have suspected that this preserves a historical core of the existence of two parties in opposition to one another, one headed by Ambrosius, and the other by Vortigern. J.N.L. Myres built upon this suspicion and put forth the hypothesis that belief in Pelagianism reflected an actively provincial outlook in Britain, and that Vortigern represented the Pelagian party, while Ambrosius led the Catholic one. Some later historians accepted this hypothesis as fact, and have created a narrative of events in fifth-century Britain with various degrees of elaborate detail. Yet a simpler alternative interpretation of this conflict between these two figures is that the Historia Brittonum is preserving traditions hostile to the purported descendants of Vortigern, who at this time were a ruling house in Powys. This interpretation is supported by the negative character of all of the stories retold about Vortigern in the Historia Brittonum, which include his alleged parctice of incest. In speculating about this conflict, one must admit that it is very likely the historical Ambrosius and Vortigern never even heard of one another during their real lives, let alone actually met.
Aurelius Ambrosianus also appears in later Arthurian legend as "Ambrosius", son of High-King Constantine, successor of Vortigern to the throne, older brother and predecessor to Uther Pendragon and the first High-King to appoint Merlin as a royal advisor. The Welsh apparently had traditions of two different Ambrosianii, whom Geoffrey of Monmouth confused.
S. Appelbaum has suggested that Amesbury in Wiltshire might preserve in it the name of Ambrosius, and perhaps Amesbury was the seat of his power base in the later fifth century. Place name scholars have found a number of place names through the Midland dialect regions of Britain with placenames incorporating the ambre- element: Ombersley in Worcestershire, Ambrosden in Oxfordshire, Amberley in Herefordshire, and Amberley in Gloucestershire. These scholars have claimed this element rerpresents an Old English word amor, the name of a woodland bird. However, Amesbury in Wiltshire is in a different dialect region, and does not easily fit into the pattern of the Midland dialect place names. This makes Appelbaum's suggestion more likely. If we combine this etymology with the tradition reported by Geoffrey of Monmouth stating Ambrosius Aurelianus built Stonehenge -- which is located within the parish of Amesbury -- and with the presence of an Iron age hill fort also in that parish, then it is extremely tempting to connect this shadowy figure with Amesbury.