One of the fascinating things about researching the legend of Arthur is that you can almost watch the story grow over time.
What turns history (or even pseudo-history) into legend is that it grows and changes in the telling and retelling. Some changes are introduced as later tellers reframe it into a world they recognize—changing Arthur into a king and his companions into knights, for example. Sometimes it grows by plain old-fashioned exaggeration. Sometimes other stories are added to it—older stories or brand new ones.
Not much more about how the legend of Arthur was growing survives until The History of the Britons in the early ninth century—more than 300 years after the Battle of Badon Hill. It’s the first historical source to actually name Arthur, describing him as a warrior and a war leader, but not a king, though it describes him fighting alongside the kings of the Britons. It lists twelve battles, most of which cannot be located from the description. Some of them are listed in other sources, but not attributed to Arthur. The History is also the first historical source to connect Arthur with the Battle of Badon Hill. Though the text also claims that Arthur killed 960 of the enemy all by himself in that battle, somewhat undermining its credibility.
It also repeats the story of Vortigern and Hengist, adding the story of Vortigern’s concessions in return for marriage to Hengist’s daughter, Rowena. And it gives the first version of the story of Dinas Emrys, where Vortigern’s attempt to build a tower on top of the hill was thwarted because the tower kept falling down. Vortigern was advised to sprinkle the blood of a boy without a father over the site to lift the curse. When Emrys is brought to him, the boy reveals that the cause is two dragons fighting underneath the hill.
The next source is the Annals of Cambria, probably written down around the middle of the tenth century, although the earliest surviving copy is from the twelfth century. It adds the detail that both Arthur and Medraut (Mordred) were killed in the Battle of Camlann. It doesn’t say whether they were fighting together or against each other, though. There’s also a disputed entry for Merlin. It’s disputed because it uses a form of his name, Merlinus, that only appeared later. The old Welsh form would have been Myrddin or Merdin.
The Welsh texts are mostly frustrating. Most survive as later copies (13th or 14th century) and so may have been changed from the original. And some of them, like the Triads of the Island of Britain and The Stanzas of the Graves, seem to be mnemonics to help oral story tellers remember details, which means they give hints, but don’t actually tell the story. But there are a few things that can be gleaned. Camlann, Arthur’s last battle is mentioned. Arthur, Gawain (Gwalchmai in Welsh), Bedevere (Bedwyr), and Kay (Cei) are also mentioned. So is Yvain (Owain), son of King Uriens of Rheged.
Up to this point, the stories about Arthur were mainly told in Britain and Brittany. But then came Geoffrey of Monmouth and his version of the Arthur story as told in the History of the Kings of Britain in the first half of the twelfth century—after the Norman Conquest. Geoffrey was writing for a new Anglo-Norman/Norman-French audience. He claimed to have had access to an ancient book, now lost. Maybe not, but he might have used some original sources. He certainly added a good amount of imagination, too.
Geoffrey’s Merlin is probably based on at least two actual characters from Welsh tradition—neither of which had any previous connection to Arthur—blended together. He also changed the name from Myrddin to Merlin to better appeal to his French-speaking audience.
Without regard to any actual history, he also has Arthur conquer most of Europe and even Rome. Possibly he conflated Arthur with a fourth-century Roman general with ties to Britannia, Magnus Maximus, who actually did some of that in the 380’s. Maximus was a Roman general stationed in Britannia and he may have married a Welsh princess. But he was not a Briton, having been born in Galicia on the Iberian Peninsula. His legions declared him emperor and he took control of Britannia and Gaul for a while, with at least grudging acceptance of the actual Roman Emperor of the time. He lasted for about four years. When he tried to invade Italy to take Rome, too, Emperor Theodosius I defeated and executed him. Magnus Maximus was definitely not Arthur.
Geoffrey of Monmouth also gives a different account of Mordred than the one we’re more familiar with. According to Geoffrey, while Arthur was in Europe, word reached him that Guinevere and Mordred were having an affair. According to Geoffrey, this is the cause of the Battle of Camlann, where Arthur kills Mordred and is mortally wounded himself. But Arthur is carried off to the Isle of Avalon to be healed.
Finally, Geoffrey completes the reframing of the story into a medieval world, rather than the actual context of the Dark Ages. Arthur is a king—in fact, he’s not just a high king, he’s an emperor. His companions are knights. And Geoffrey introduces the very medieval concept of courtly love, as well.
The older versions of the Arthur story were mainly about his battles. But Geoffrey’s version took off, making the legend known—and improved upon—across Europe. After The History of the Kings of Britain, the legend snowballs. Wace adds the Round Table and the idea that Arthur might return some day.
And then Chrétien de Troyes gets hold of it. This is where we get most of the romances. He adds Lancelot, the greatest knight in the world—who just happens to be French. And he starts the story of the adultery of Lancelot and Guinevere. He also adds Perceval and the Grail Quest.
More and more gets added and changed over time, but that’s where the core of what we know as the legend of Arthur comes from.