Nearly all of our knowledge of the history of the Britons during the Roman and Sub-Roman periods comes from chronicles written by historians writing during the medieval period. This is a problem, because these chronicles are laughable at best.
Geoffrey of Monmouth's Historia Regum Britanniae and the earlier Historia Britonum on which it is partly based give us an implausibly precise list of the kings of Britain, beginning with Brutus of Troy, a refugee from the Trojan war whose line was, apparently, unbroken through 99 subsequent kings (if one counts the winning factions of two brief succession crises and their attendant interregnums), with the hundredth King of the Britons being Vortigern; Geoffrey's list continues after Vortigern for another dozen reigns as the Saxons expand their hold on Britain. A hundred generations is, of course, implausible; the oldest continuous chain of succession in the modern world is the ownership of a hotel in Japan that dates to the early 8th century CE, which means it's been in operation for about a tenth of the history of settled people. By tracing the kingship of Britain from the final monarch, Cadwallader, who died in the 7th century, back to Brutus in the 12th century BCE, Geoffrey asserts a system that mostly survived for two thousand years.
Vortigern, however, is an interesting monarch to mention. Geoffrey of Monmouth concurs with the anonymous Anglo-Saxon Chronicle that this is where things started to go downhill for the Britons.
Vortigern, we are told in both sources, was at his wits' end dealing with the Roman occupiers in his realm, and contacted Hengest and Horsa, twin Jutnish leaders (their names mean "Stallion" and "Horse") to bring people from continental Europe to serve as mercenaries. This is a slightly strange decision, as the Roman empire had ceased to rule Britain from Rome three kings ago; it seems, however, that the Romans on the British Isles did not depart as their empire lost its ability to control the islands, and so the native lords saw fit to bring fighters from elsewhere.
Hengest and Horsa established the first Anglo-Saxon kingdom on the British Isles, and from there they and their descendants continued to expand their grasp. Meanwhile, among the native Britons (whose language, by the way, was an ancestor of modern Breton and Welsh), Vortigern was followed by Vortimer, whose successor was Aurelius Ambrosius - probably a historical ruler, and certainly culturally a Roman. Remember his name, for we will visit him in later posts. We will also visit his putative successor: Uther Pendragon.
The historicity of Hengest and Horsa is in doubt. Horsa is especially dubious; the semantic similarity between the twins' names has led some to suggest that they are a doubling of a single warlord. There's also an association between horses and kingship throughout the earlier Indo-European world, so it's possible the names were added later to legends taken from a real conquering lord or pair of lords. But in any case, it is clear that the Angles, Saxons, and Jutes invaded Great Britain, forcing the Celtic Britons (or at least their ruling class) out of power in England; Cadwaldr, the historical source for the last King of the Britons, has stories told of his attempts to repulse the invaders from Wales, stories which give rise to Welsh festival observances that continue to the present day.
While the slow expansion of the Anglo-Saxons into formerly British territory is well-documented in archaeology, we have no way to fact-check Geoffrey of Monmouth's sequence of kings. And, given that he traces it in a continuous arc all they way from Brutus to Cadwallader, finding that it checked out would be shocking.