Nennius was a Welsh scholar who lived and worked in the ninth century. He is traditionally credited as being the author of the important historical document the Historia Brittonum [History of the Britons], a brief history of Britain from Roman times to the 680s, with assorted supplementary material (including an account of the Ages of the World and various genealogies). The Historia is celebrated as being the earliest source to mention (King) Arthur, who is presented as a powerful British war-lord who fought a series of successful battles against the invading Anglo-Saxons (in the late fifth century).
In fact, Nennius was one of a number of redactors of the Historia Brittonum, which had been composed …
Magennis, Hugh. "Nennius". The Literary Encyclopedia. First published 28 August 2002
[http://www.litencyc.com/php/speople.php?rec=true&UID=3320, accessed 28 February 2017.]