The earliest literary references to Arthur are found in early Welsh poetry.

He is mentioned briefly in the late 6th century Welsh poem cycle The Gododdin, attributed to the poet Aneirin. In one verse, the bravery of one of the warriors is described fulsomely; whoever this warrior was great "though he was not Arthur".

Since these poems are known only from a manuscript of the 13th century, it is impossible to determine whether this passage is original or a later interpolation. Several poems attributed to Taliesin, a bardic-poet said to have lived in the 6th century, refer to King Arthur, including The Chair of the Sovereign, which refers to "Arthur the Blessed". Another poem called The Treasures of Annwn recounts a journey of Arthur into the Celtic Otherworld. The Journey to Deganwy, stated that: "as at the battle of Badon, with Arthur, chief holder of feasts, his tall blades red from the battle all men remember".

King Arthur also appears in a number of well known vitae or Lives of 6th and 7th century Christian saints. Most of these Lives were written at Llancarfan monastery in the 12th century. In the Life of Saint Illtud, possibly written around 1140, Arthur is said to be a cousin of Saint Illtud. And according to the Life of Saint Gildas, written in the 11th century by Caradoc of Llancarfan, Arthur killed Gildas' brother Hueil, who was a marauding pirate, on the Isle of Man.

In the Life of Saint Cadoc, written around 1100 by Lifris of Llancarfan, Saint Cadoc gives protection to a man who killed three of Arthur's soldiers. In return, King Arthur demands a herd of cattle as wergeld for his men. Cadoc delivers the cattle as demanded, but when King Arthur takes possession of the animals they change into bundles of ferns.

Another account appears in the Legenda Sancti Goeznovii, which portrays the king with no obvious touches of legend or fantasy. More magical incidents are described in the late medieval biographies of Carannog, Padern, and Efflam.

An early Welsh poem found in the Black Book of Carmarthen, called Pa gur yv y porthaur? or What man is the gatekeeper?, is a dialogue between Arthur and the gatekeeper of a castle he wishes to enter.

To gain entry, King Arthur recounts the deeds of his men, notably Cai and Bedwyr. Other Welsh tales about King Arthur, such as Culhwch and Olwen, are included in the Mabinogion collection.
 

Gildas

The earliest mention of King Arthur is found in the work of Gildas (c.493 - 570). In De Excidio et Conquestu Britanniae (c.540) we have the earliest portrait of someone who may be King Arthur.

He is presented as being a British commander of Roman descent repelling Saxon invaders at the siege of Mons Badonicus or the Battle of Badon in about AD500. Gildas also depicts an unnamed leader, probably Ambrosius Aurelianus, whom Gildas elsewhere praises and who becomes Arthur's uncle in later chronicles.

Nennius

Writing in the 8th Century AD, Nennius in Historia Britonum (c.769) gives a mythical account of the origins of the Britons. He also has the first direct mention of King Arthur, by name, as a Christian warleader, who won twelve battles against the pagan Saxons.

Nennius actually calls King Arthur "dux bellorum" which means 'commander of battles' or 'warleader'.

The last of these great battles is fought at Mons Badonis or the Battle of Badon. Nennius relates the story of the British King Vortigern and a marvellous boy, named Ambrosius, the original Merlin.