I KNOW MANY OF BRITAIN’S HIDDEN HISTORY FORUMS USERS will know of this, but my searches on the Comet Lead to: Geoffrey’s Historia and the Brut y Brenhinedd and much more.
Try searching Geoffrey’s Historia Regum Britanniae (completed by c. 1139) purports to narrate the history of the Kings of Britain from its eponymous founder Brutus of Troy to Cadwaladr, the last in the line. Geoffrey professed to have based his history on “a certain very ancient book” written in britannicus sermo (the “British tongue”, i.e. Common Brittonic, Welsh, Cornish or Breton) which he had received from Walter of Oxford.
It became one of the most popular works in the medieval West, but its impact was particularly profound and enduring in Wales, where the Historia was accepted as a largely authentic and authoritative account. The influence is most clearly evidenced by the existence of several translations into Welsh from the 13th century onwards, usually known as Brut y Brenhinedd. The manuscript history of these texts is a rich and long one attesting to the production of several translations and new redactions, most of which were copied many times over.
The Welsh renderings are not straightforward translations in the modern sense, but by contemporary standards, they are generally close to their Latin source text, with only some commentary or additional material from bardic traditional lore (cyfarwydd) appended to the text.