Also Arthur himself, having put on a coat of mail suitable to the grandeur or so powerful a king, placed a golden helmet upon his head, on which was engraven the figure of a dragon; and on his shoulders his shield called Priwen; upon which a picture of the blessed Mary, mother of God, was painted, in order to put him frequently in mind of her. Then girding on his Caliburn, which was an excellent sword made in the isle of Avallon, he graced his right hand with his lance, named Ron, which was hard, broad, and fit for slaughter.
While it’s not the earliest work to reference Arthur, the 1136 History of the Kings of Britain by Geoffrey of Monmouth is the earliest surviving canon - in which I mean that it gives a full enough picture of what the basic plot points of the character and tale are for later writers to hang their coats on1.
As the name suggests, it’s a history - or rather a pseudohistory since it takes a very loose approach on reality - of a whole series of kings, from the Trojan War to the last major Brythonic king Cadwallader. This means it gives a very comprehensive history before Arthur’s reign2, though King Arthur fills up a decent chunk of it.
This is the work that introduces Merlin3 and lays out most of the basic bones of the Arthurian narrative - his birth, rise to kingship, battle of Badon, marriage, death by Mordred, etc. - though many characters and events that will be famous later (such as Lancelot) are not yet present.
Because it’s relatively complete and early, without time for as many anachronisms to slip in4, it’s often used as a source for retellings that want to capture “the original Arthur” - or just to fill in elements that are left off from later works.
Footnotes
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And while some earlier works may be able to have done this to other sources we no longer have, or to a broader oral tradition, its early presence and wide popularity means even the oral tradition of any meaningful stories we have written down were influenced by its backwash if nothing else, long before our records of them. The only truly pre-Galfridian pieces we have left, and which practically anyone outside the Middle Ages had, are so fragmentary you can’t put together a complete story with them. ↩
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Including characters which tend to major in Arthurian works lingering on the pre-Arthur period, such as his father Uther, the wicked earlier king Vortigern, etc. - and a few stories which have survived and not gotten fully absorbed by the legend, like King Lear. ↩
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Fairly clearly cribbed from two previous characters that we have from older works, merged together. ↩
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As later writers imagine Arthur closer to their own times. The Historia is still 5 centuries after the time it purports Arthur to have lived in, but this is the closest we can get. ↩