Historians are not in agreement as to whether or not the ‘real’ Arthur—the living, breathing, fighting human being—ever existed. The original sources for the legend of King Arthur come from a few Welsh texts. These are:
1) Y Goddodin—a Welsh poem by the 7th century poet, Aneirin, with it’s passing mention of Arthur. The author refers to the battle of Catraeth, fought around AD 600 and describes a warrior who “fed black ravens on the ramparts of a fortress, though he was no Arthur”. http://www.missgien.net/celtic/gododdin/poem.html
2) Gildas, a 6th century British cleric who wrote De Excidio et Conquestu Britanniae (On the Ruin and Conquest of Britain). He never mentions Arthur, although he states that his own birth was in the year of the siege of Mount Badon. The fact that he does not mention Arthur, and yet is our only historian of the 6th century, is an example of why many historians suspect that King Arthur never existed. http://www.fordham.edu/halsall/source/gildas.html
3) Taliesin, a 6th century poet, to whom The Spoils of Annwn, is ascribed. This poem is only one of several in which he mentions Arthur. http://www.maryjones.us/ctexts/t30.html
4) Nennius – “History of the Britons” (Historia Brittonum, c. 829-30)
“Then it was, that the magnanimous Arthur, with all the kings and military force of Britain, fought against the Saxons. And though there were many more noble than himself, yet he was twelve times chosen their commander, and was as often conqueror.” http://www.fordham.edu/halsall/basis/nennius-full.html
5) Native Welsh Tales: These connected works of Welsh mythology were named the Mabinogion in the 19th century by their first translator, Lady Charlotte Guest. These include the story of Culhwch and Olwen, in which Arthur and his men track down the thirteen treasures of Britain, and The Dream of Rhonabwy. These stories are found in the Red Book of Hergest and/or the White Book of Rhydderch, both copied in the mid-14th century. http://www.maryjones.us/ctexts/index_welsh.html
6) The Annales Cambriae. This book is a Welsh chronicle compiled no later than the 10th century AD. It consists of a series of dates, two of which mention Arthur: “Year 72, The Battle of Badon, in which Arthur carried the cross of our Lord Jesus Christ on his shoulders for three days and three nights and the Britons were victors. Year 93, The Strife of Camlann in which Arthur and Medraut fell.” The early dates of the above works indicate little or no relation to the later English/French embellishments of Arthur, which Geoffrey of Monmouth popularized. http://www.fordham.edu/halsall/source/annalescambriae.html
Later texts that are built on the above works, in chronological order, are:
1) William, Chaplain to Bishop Eudo of Leon – “Legend of St. Goeznovius, preface” (c. 1019)
“In the course of time, the usurping king Vortigern, to buttress the defence of the kingdom of Great Britain which he unrighteously held, summoned warlike men from the land of Saxony and made them his allies in the kingdom. Since they were pagans and of devilish character, lusting by their nature to shed human blood, they drew many evils upon the Britons. Presently their pride was checked for a while through the great Arthur, king of the Britons. They were largely cleared from the island and reduced to subjection. But when this same Arthur, after many victories which he won gloriously in Britain and in Gaul, was summoned at last from human activity, the way was open for the Saxons to go again into the islane, and there was great oppression of the Britons, destruction of churches and persecution of saints. This persecution went on through the times of many kings, Saxons and Britons striving back and forth. In those days, many holy men gave themselves up to martyrdom; others, in conformity to the Gsopel, left the greater Britain which is now the Saxon’s homeland, and sailed across to the lesser Britain [ed. note: Brittany].”.]
[ed. note from Brittanica.com: There are enough similarities with Geoffrey's "History" that some have questioned whether Goeznovious might be of later date, i.e. post-Geoffrey. But, unless William's original source, "Ystoria Britannica," is found and proves otherwise, we have to consider the possibility that Geoffrey may have used Goeznovious as a source.
2) William of Malmesbury - "The Deeds of the Kings of England (De Gestis Regum Anglorum)" (c. 1125)
"When he [ed. note: Vortigern's son, Vortimer] died the strength of the Britons diminished and all hope left them. They would soon have been altogether destroyed if Ambrosius, the sole survivor of the Romans who became king after Vortigern, had not defeated the presumptuous barbarians with the powerful aid of the warlike Arthur. This is that Arthur of whom the trifling of the Britons talks such nonsense even today; a man clearly worthy not to be dreamed of in fallacious fables, but to be proclaimed in veracious histories, as one who long sustained his tottering country, and gave the shattered minds of his fellow citizens an edge for war.
3) Henry of Huntingdon – “History of the English” (Historia Anglorum, c. 1130)
“The valiant Arthur, who was at that time the commander of the soldiers and kings of Britain, fought against [the invaders] invincibly. Twelve times he led in battle. Twelve times was he victorious in battle. The twelfth and hardest battle that Arthur fought against the Saxons was on Mount Badon, where 440 of his men died in the attack that day, and no Briton stayed to support him, the Lord alone strengthening him.”
4) The History of the Kings of Britain, by Geoffrey of Monmouth, dating to the middle 12th century. This is the beginning of the King Arthur legend as we know it. Geoffrey was born in Wales, but worked for his patron, Robert of Gloucester, who was particularly interested in legitimizing the claim of his sister (Matilda) to the English crown. Thus, the confusion of landmarks which moved Arthur from Wales to England proper, and the romanticizing of the tale, including the notion that Britain was originally conquered by Brutus, the son of the Trojan hero Aeneas, and thus Britain was ‘classical’ in origin.
5) Roman y Brut (The Romance of Brutus) is the translation of Geoffrey’s work into Anglo-Norman verse. It takes much of Geoffrey’s story and adds the round table, courtly love, and chivalry, thus transforming Arthur from a Welsh warrior to a medieval, Anglo-French knight. From this point, the Welsh Arthur is all but lost, and the Anglo/Norman/French ‘King Arthur’ is paramount.
By 1191, the monks of Glastonbury were claiming knowledge of his grave, and soon after, the link between Arthur and the Holy Grail, which Joseph of Arimathea supposedly brought there. By 1225, monks in France had written The Vulgate Cycle, telling of the holy grail from the death of Jesus Christ to the death of Arthur, and included the romance of Lancelot and Guinevere. This story became the standard version used throughout Europe.
One critic stands out, however: William of Newburgh – “History of English Affairs” (Historia rerum Anglicarum, c. 1198)
“For the purpose of washing out those stains from the character of the Britons, a writer in our times has started up and invented the most ridiculous fictions concerning them, and with unblushing effrontery, extols them far above the Macedonians and Romans. He is called Geoffrey, surnamed Arthur, from having given, in a Latin version, the fabulous exploits of Arthur, drawn from the traditional fictions of the Britons, with additions of his own, and endeavored to dignify them with the name of authentic history.”
[ed. note: Amid the near universal chorus of hosannas heard throughout Europe for Geoffrey of Monmouth and his "History of the Kings of Britain," William of Newburgh stands out as, perhaps, the first and certainly his most ardent critic. In fact, the full preface to his 'History' is taken up with ever-crescendoing criticsm, of which the above quote is only the opening salvo. CLICK HERE to read William of Newburgh's full preface.] http://www.britannia.com/history/arthur/historians.html