Gildas

"Of Gildas the Wise concerning the destruction and conquest of Britain, and his lamentable castigation uttered against the kings, princes and priests thereof."

Gildas Bandonicus, a British [i.e. Celtic] monk, lived in the 6th century in the South-West of Britain. In the 540s - in the most aggressive language - he set out to denounce the wickedness of his times.

His book, Concerning the Ruin of Britain or De Excidio Britonum, is the only substantial source which survives from the time of the Anglo-Saxon conquest of Britain and is the only contemporary Arthurian source that can be examined today. 

 The author himself in his opening words describes his work as an epistle. For ten years it has been in his mind, he says, to deliver his testimony about the wickedness and corruption of the British state and church; but he has, though with difficulty, kept silence. Now, he must prove himself worthy of the charge laid upon him as a leading teacher, and speak. But first, he will, with God’s help, set forth shortly some facts about the character of the country and the fortunes of its people. 

In compiling it, Gildas says he has not used native sources, which, if they ever existed, had perished, but "narratives from beyond the sea." What this precisely means it is not easy to determine. The story, as he tells it, clearly appears to be derived from oral traditions  rather than copied from any older written sources. Brief and rather vague as it is, the narrative may be accepted as representing truly enough the course of events.

The Anglo-Saxons began arriving in the 470s, perhaps imported as soldiers as Gildas suggests. For some time the British fought back [the historic basis of the Arthurian myth], but by 600 the Anglo-Saxons had control of most of what becomes 'England', and the Celtic peoples were pushed to the hills of Wales and Scotland and across the English Channel to "Brittany".

  It brings us down to the time, forty-four years after the British victory of Mount Badon, when the descendants of the hero of that field, Ambrosius Aurelianus, had departed from the virtues of their great ancestor, and when, in the view of our author, the moral and spiritual state of the whole British dominion had sunk to the lowest level of degradation. In the pages that follow, he attacks, successively and by name, five of the princes of the west: Constantine of Devon and Cornwall, Aurelius Caninus, whose sphere of influence is unknown, Vortipor of Pembrokeshire, Cuneglasus, king of an unnamed territory; and the "dragon of the isle," Maglocunus, who is known to have reigned over Anglesey and to have died in the year 547. Each of these is savagely reproached with his crimes—sacrilege, perjury, adultery and murder—and each is, in milder terms, entreated to return to the ways of peace.

Read De Excidio Britonum