Vortigern was a king of Britain who has entered into legend as a proud tyrant. Historically speaking, he seems to have been the first ruler of Britannia after the Romans left in 410 AD, though records from this period are very scanty as civilisation was collapsing everywhere, hence the name 'Dark Ages'.
According to Nennius, Vortigern rose to power around 425 AD, prior to this Britain seems to have been run by a council of magistrates. Vortigern was a follower of Pelagianism, a heresy that denied original sin and preached salvation by one's own efforts, rather than those of any outside agency. His opponents were the Roman Catholics, and it wasn't long before civil war broke out in Britain between these two factions. In light of this civil war and increased threat of attack from the Picts, Irish and Anglo-Saxons, Vortigern employed Saxon mercenaries under Hengist as his allies, but these turned against him and slaughtered his men. Hengist is said to have then founded the Anglo-Saxon Kingdom of Kent, in the extreme South East of Britain. This, being the first Germanic Kingdom on the island of Great Britain, is part of the founding mythology of the English. Vortigern himself fled to the mountains of Wales. The leader of the Roman Catholic faction Ambrosius Aurelianus, became king in Vortigern's place.