The King Arthur of Arthurian lore (student of Merlyn, ruler of all of Britain, leader of the round table, creator of an idealistic society called Camelot) almost certainly did not exist. There is no historical record of such a man, and the record of someone of such importance would not have been erased.
However, it is possible that there was a real person behind the legend of King Arthur. The two most plausible suggestions are that the “real Arthur” was either a general in the Roman legion stationed in Britain or a Welsh warrior during the time of the Saxon invasions in the 5th Century.
With a little imagination, it is possible to see how the legend of King Arthur could have started with such a man. The fable of a king who kept the peace could have been started by either man. A great Roman general would have defended Britain from the invading tribes of Scotland. A great Welsh warrior would have defended the people in what is now Wales from the Saxons.
It is also possible that Alfred the Great, an Anglo-Saxon king of Wessex, who united several kingdoms within Britain, is the historical source for the King Arthur legend.
There are also other pieces of the legend that might have basis in history. There was most likely an historical figure who inspired the character of Vortigern. In Arthurian literature, Vortigern is a corrupt leader who is usually either a contemporary/rival or predecessor of Uther, Arthur’s father.
There is an historical record of an overlord called Vortigern, who lived in the 5th Century. He was a corrupt leader who contracted out the defence of Britain from the Picts and the Scots to Saxon mercenaries living across the English channel. In exchange for their services, he gave the Saxons grain tributes and prime lands that had been occupied by Briton lords. Eventually, he could not reconcile the competing demands of the Saxons with those of the other Britons, and the Saxons killed him when he refused to pay the tribute owed (The History Channel).
Some scholars believe that the ruins of an ancient fort in modern-day Wales is the remnants of Vortigern’s primary base. It overlooks the river Teifi, an area which Welsh historical sources connect to Vortigern.
One factor complicating the historical record regarding Vortigern, is that the name wasn’t a surname, but a title. Vortigern’s loose translation was high lord or high king (The History Channel).
Another problem complicating the historical record regarding Vortigern is that Welsh historical sources were intermingled with legends. Therefore, it is often difficult to parse out fact from fiction.
“Barbarians: The Saxons.” The History Channel. 2007.