The Arthurian saga we know from writers like Sir Thomas Malory, and films like “Excalibur” is certainly a myth.
Elements like the magic sword, the Round Table and the Grail quest can all be found in old Celtic tales that contributed to the Arthurian story. However, there may well have been a real-life source for it. If the figure of King Arthur has any foundation in fact, the likeliest time for him to have lived is the early 5th century, after the Romans left and before the Saxon invasion fully took hold.
There are persistent legends from around that time of a British (Celtic) chieftain, trained in the Roman arts of war, who united the warring British tribes to fight off the Saxons. In fact one 9th-century historian firmly states that there was such a man, and his name was Arthur.
However, the era in question is so confused and its records so incomplete that the historian, Nennius, can’t have been sure. There certainly was a chieftain called Ambrosius Aurelia-nus, who may be the source of the story. We shall probably never know for sure.