>>14975352
From Brythonic/Welsh folklore(there's a chance that it came from Straithclyde, I don't care enough to recall the exact characters that may have been "northern Arthurs" though), altho there's no preserved full celtic version of the cycle, Mabinogion has some really old takes on Arthur in "Culhwch and Olwen" and so on, it's probably just the oral tradition that's older than 12th century and given that Mabinogion was written in prose it's likely that no anachronisms have been preserved in the text(I don't know anything about that side of it though). My take is that the actual historical Arthur defeated Anglo-Saxons at some point and died in battle against a man who's name was close to Mordred, everything else is probably just an attempt to link various families etc. to the legendary hero.
The first complete cycle can be found in "Merlin and the Grail", most of the later recollections are based of "The Death of Arthur" though(as it was written down as a kind of full-blown collection of Arthurian cycles, with lengthy digressions describing the actions of different knights).