Nennius, the lowly minister and servant of the servants of God, by the grace of God, disciple of St. Elbotus, to all the followers of truth sendeth health
Thus begins the Historia Brittonum, "History of the Britons." The survival of about three dozen manuscripts tells how popular it was.
Nennius was a Welsh monk and historian who flourished about 800. The "Elbotus" he mentions in his opening line refers to Bishop Elfodd of Bangor, who persuaded the Welsh Christian church to accept the Roman method for computing Easter.
The Historia is a compilation of other sources, some of which (such as Bede, and Gildas' De Excidio et Conquestu Britanniae, "On the Ruin and Conquest of Britain") are obvious. Other sources are not clearly identified, but it is highly unlikely that Nennius was making things up. His goal would be to bring together several sources; some of them may be oral histories for which we don't have other evidence, including a-historical legends and folklore.
One example of non-provable detail is the legend that Britain was founded by Aeneas after leaving Troy. Another is Nennius' contribution to the Arthurian legends by listing his twelve battles, the geographical locations of which have challenged historians. He also calls Arthur dux bellorum, "duke/leader of battles" rather than a king.
The various manuscripts have many differences: having been made by hand, there are omissions in some of individual words or whole paragraphs, and the scant autobiographical material in it is not consistent. Nennius' authorship has been questioned, but since it is the only name associated with the majority of manuscripts, Nennius still gets the credit.
Speaking of giving credit to historians, I keep referring to Gildas without really telling you who he was or why he's considered important. He, too, added much to the legend of Arthur in his history, which we will turn to next time.