More likely than not someone who briefly resurrected the order of Rome after 410AD. Probably tied into the entire celtic mythology though and thus magnified over time by various literary sources.
Do any of them, or all of them, compose a Historical King Arthur, though?
I am not yet born, console me.
I fear that the human race may with tall walls wall me,
with strong drugs dope me, with wise lies lure me,
on black racks rack me, in blood-baths roll me.