Historia Brittonum

The Historia Brittonum (History of the Britons), which evidently originated in early-9th century Wales, has survived, in several versions (recensions), in more than thirty medieval manuscripts. The principal, i.e. fullest, text is to be found in Harleian MS 3859 (in the British Library, London) – which also contains the A-text of the Annales Cambriae and the earliest Welsh collection of royal genealogies. The Harleian manuscript dates from c.1100, but internal evidence suggests that the Historia text was originally compiled c.829.[*] Traditionally, on the strength of a prologue that features in just a handful of manuscripts (Harleian MS 3859 is not one of them), the work has been attributed to Nennius[*]:

I, Nennius, pupil [discipulus] of Elvodugus, have undertaken to write down some extracts that the stupidity of the British nation cast out; for the scholars of the island of Britain had no skill, and set down no record in books. I have therefore made a heap of all that I have found, both from the annals of the Romans and from the chronicles of the Holy Fathers – that is Hieronymus, Eusebius, Isidore and Prosper – and from the annals of the Scots [i.e. Irish] and the Saxons, and out of the tradition of our elders. Many learned scholars and copyists have tried to write, but somehow they have left the subject more obscure, whether through repeated pestilence, or frequent calamities of war. I ask every reader who reads this book to pardon me for daring to write so much here after so many, like a chattering bird or an incompetent judge. I yield to whoever may be better acquainted with this skill than I am.[*]

None of the surviving manuscripts of this, so-called, ‘Nennian recension’ is earlier than the 12th century,[*] though a text of the recension was translated from Latin into Middle Irish during the later-11th century.[*] The attribution to Nennius is now generally regarded as an 11th century invention.[*] Nevertheless, “a heap” might be considered a reasonable description of the miscellaneous assortment of material, riddled with errors and inconsistencies, presented by the Historia Brittonum.

The contents of Harleian MS 3859 can be categorized as follows –
§§1–6: The Six Ages of the World.   §§7–18: British, Irish and Pictish origins.   §§19–30: Roman Britain.   §§31–49: The Adventus Saxonum story.   §§50–55: Life of St Patrick.   §56: Arthur’s battles.   §§57–61: Genealogies of Anglo-Saxon kings.   §§62–65: Northumbria.   §66: Chronological computations.   §66a: Cities of Britain.   §§67–76: The Wonders of Britain, Anglesey and Ireland.[*]

Though it is clearly intended to provide a survey of the past history of the British it is an ill-synthesized and ill-digested work; the compiler gathered together and transcribed more or less relevant documents of very different types. Its contents, therefore, are very varied …
Wendy Davies Wales in the Early Middle Ages (1982) Appendix

Indeed, a good deal of the Historia is clearly fantasy.

Much of the work consists of passages which, save that they lack the introduction ‘once upon a time’, have all the historical reliability of fairy-stories.
A.S. Esmonde Cleary The Ending of Roman Britain (1989) Chapter 5

On the other hand, it must, so the theory goes, also contain historical nuggets.

Considerations of plausibility, the Historia’s relation to earlier sources, its chronology and compatibility with the archaeological evidence – all these combine to suggest that the Historia does contain genuine early information, albeit in a very adulterated form.
Michael E. Jones The End of Roman Britain (1996) Appendix 4
The problem is that an objective method of sorting the wheat from the chaff has never been forthcoming; you pays your money and you takes your choice of what commends itself to you at the time.
A.S. Esmonde Cleary The Ending of Roman Britain Chapter 5
It is impossible, therefore, to generalize about the value of the Historia Brittonum. On the one hand it provides first-hand evidence of the attitude to history writing of a Welshman of the early ninth century. On the other it presents us with edited documents of varying dates and provenances whose origins are not yet sufficiently understood; at the least, however, the latter indicates some of the materials that were available to the historian in the early ninth century.
Wendy Davies Wales in the Early Middle Ages Appendix
The Historia notes (§4) that “From the Passion of Christ seven hundred and ninety-six years have been completed; from his Incarnation, eight hundred and thirty-one.” This is inconsistent. David Dumville* argues that, although numbers in Harleian MS 3859 are written out in words, they were in Roman numerals at a previous stage in the text’s transmission, and that dcccxxxi (831) is a scribal error for dcccxxix (829), which would reconcile the Passion and Incarnation dates. Also, there is (in §16) a reference suggesting that at the time of writing it was the fourth year of Mermini regis, i.e. Merfyn Frych, king of Gwynedd. By juggling with various dating references in the Historia, David Dumville arrives at a date of 830 for Merfyn’s fourth year. The Annales Cambriae indicate that Merfyn’s predecessor, Hywel, died in 825. So the indications are that the Historia text was composed c.829.
* ‘Some Aspects of the Chronology of the Historia Brittonum’, Bulletin of the Board of Celtic Studies Vol. 25 (1972–4).
There are five manuscripts of the ‘Nennian recension’. The greatest number of manuscripts (over twenty) spuriously attribute the Historia to Gildas, hence they are the ‘Gildasian recension’. The principal representative of the ‘Vatican recension’ (Vatican MS Latin 1964), which comprises four manuscripts, attributes the work to “Marcus the Anchorite, a holy bishop of that people [i.e. the Britons]”. The now-lost manuscript of the ‘Chartres recension’ (it was destroyed during World War II) talked of “excerpts of the son of Urien found in the Book of Saint Germanus”. The four manuscripts of the ‘Harleian recension’ (named after Harleian MS 3859) claim no authorship.
In terms of manuscript age, the Vatican manuscript (later-11th century) predates the Harleian, and the now-lost Chartres manuscript (Chartres Bibliothèque Municipale MS 98) was probably also 11th century. In fact, the Historia manuscript was not part of the actual contents of the Chartres codex. The text was not complete – ending mid-sentence, mid-page – and the manuscript had been broken-up to provide the codex’s flyleaves.
David Dumville’s PhD thesis (University of Edinburgh, 1975), ‘The Textual History of the Welsh-Latin Historia Brittonum’, is freely available online.
The earliest extant representative of the ‘Nennian recension’ (Corpus Christi College, Cambridge, MS 139) can be precisely dated 1164. The other four manuscripts are descended from this one. In fact, the name Nennius in the famous prologue (above) is actually spelled Ninnius in the Corpus Christi manuscript.
Probably later-10th century – during the reign (in Deheubarth, 950–988) of Owain ap Hywel Dda.
The Irish text is called the Lebor Bretnach, but is frequently referred to as the ‘Irish Nennius’.
Elvodugus (Elfoddw) is described as “archbishop in the Gwynedd region” in his Annales Cambriae obituary (the year 809).
David Dumville has studied the Historia extensively, and his opinion has been influential. He concludes a paper titled ‘ ‘Nennius’  and the Historia Brittonum ’:
Of this, the so-called ‘Nennian’ recension of the Historia Brittonum, we can now speak with some degree of certainty. It is a work which dates only from about the middle of the eleventh century; the attribution of the Historia to Ninnius, or ‘Nennius’ as he is traditionally known, is no earlier than this date. In terms of the textual tradition of the Historia it is a secondary development. The primary text of the Historia is represented by the anonymous ‘Harleian’ recension; we must admit to ignorance of the name of its ninth-century author.
This paper was published in Studia Celtica 10/11 (1975/6), and also incorporated into David Dumville’s PhD thesis (University of Edinburgh, 1975), freely available online.
Adapted from the translation of John Morris.
The section numbers are as per Theodor Mommsen’s edition (1898). The A-text of the Annales Cambriae and Welsh royal genealogies are interpolated between §66 and §66a.