The name given to the individual believed to have been the leader of the Romano-British administration from around 425 to 445 AD. Vortigern the name, is an anglicised version of the Welsh Guorthigirn or Gwrtheyrn, which literally means "Overlord". Which has led some to speculate that "Vortigern" was purely a title, but more likely an adopted name, signifying a political claim to power.
What we think we know about him
He is generally identified with Gildas’ "superbus tyrannus" or proud tyrant who together with "all the councillors" is responsible for decision to hire the Saxon foederati to fight the Scotii and Picts and therefore responsible for the adventus Saxonum and all its unfortunate consequences.
Although this policy was initially successful, the Saxon foederati subsequently revolted and began seizing British territory for themselves. At which point in time Vortigern lost his grip on power
His powerbase appears to have been in Powys and the Midlands. His capital may have been at Viroconium or modern day Wroxeter. (There is certainly archeological evidence of 5th century building and the grave of a wealthy Romano-Briton located at Viroconium.)
What we might reasonably conjecture about him
(1) In terms of his politics
He would have come to power as the result of the revolt of 409 AD and it is very likely that there remained acute political differences between those that remained Roman loyalists and those that sought British independence.
The Historia Brittonum informs us that Vortigern,
had cause of dread, not only from the inroads of the Scotii and Picts, but also from the Romans, and his apprehensions of Ambrosius
Which should remind us that whereas we can now see that the Western Roman Empire
(itself virtually extinguished by 476 AD) would never be in the position to re-occupy Britain
, this was by no means so obvious in Vortigern's time.
Although the decision to hire Germanic foederati was very much in keeping with standard Roman practice of that time, their settlement in Kent is clearly designed to block the traditional Roman cross-channel invasion route, and it is very probable that their deployment was as much to do with strengthening Vortigern's domestic power as it was with protecting Britain from external enemies. All of which would suggest that Vortigern was an advocate of British independance.
Some also credit him with the decision to relocate Cunedda and his Votadini to north Wales to counter the invading Scotii. Although we cannot be certain, this idea does have the merit of suggesting that he was following a consistent policy of strengthening the weak points of Britain.
His "apprehensions of Ambrosius" suggest that this Ambrosius (who we can reasonably assume to be the same Ambrosius Aurelianus as Gildas mentions) was the leader of the pro-Roman party. The Historia Brittonum suggests there was civil conflict with this Ambrosius fighting and losing to one Vitallinus at Guollopum sometime before 440 AD. (And it is suggested that Vitallinus and Vortigern were one and the same.) Although we might quibble about the details, it seems entirely reasonable to presume that the conflict between the two parties bubbled over into warfare.
(2) In terms of his religion
It has been suggested that Pelegianism, itself the creation of a Briton, had political undertones and that Vortigern was in the pro-Pelegian party. We know that support for Pelegianism in Britain was sufficiently strong to warrant the intervention of the Gallic bishop Germanus and Vortigern is traditionally portrayed as his opponent.
In truth it is difficult to say what role the Pelegian heresy played in these political disputes. Gildas, who was not one to miss the opportunity to traduce any Romano-British ruler singularly fails to accuse Vortigern of heresy. (In fact he doesn't accuse Vortigern of much at all other than the folly of inviting in the Saxons.)
Things we are told that we might be doubtful about
The Historia Brittonum gives us more information on Vortigern.It names his sons as Vortimer, Catigern, Pascent and Faustus, as well as including the claim that Faustus is the product of an incestous relation between Vortigern and his daughter. The characters of Hengist and Horsa feature as Vortigern's nemesis, treacherously slaying his supporters, taking him prisoner and only granting him his freedom in return for most of south-east Britain.
What might well have happened to him
The Historia Brittonum also gives us two version of his demise, the second of which states that Vortigern,
being hated by all the people of Britain, for having received the Saxons, and being publicly charged by St. Germanus and the clergy in the sight of God, he betook himself to flight; and, that deserted and a wanderer, he sought a place of refuge, till broken hearted, he made an ignominious end.
All of which sounds quite reasonable in the circumstances.
Part of the Sub-Roman Britain project, where sources are detailed.