Llyfr Du Caerfyrddin (the Black Book of Carmarthen) is commonly attributed to several scribes writing in the twelfth and early thirteenth centuries. Other scholars consider it to be the work of a single author working at the Priory of St John at Carmarthen around 1250CE. It has 108 folios each 7 inches by 5. It contains 38 items including: the Englynion y Beddau (Stanzas of the Graves), Gereint fab Erbin, some religious verses and the "Merlin" poems. The manuscript is made up of eight gatherings of stout vellum sewn together and bound to form a volume of 54 folios (108 pages). It is not complete for there are a number of folios missing. There is a lack of uniformity, both in the way the scribe has ruled each page before writing and in his handwriting itself, inconsistencies not generally found in manuscripts of the period written by professional scribes. He writes the first twenty folios in large letters, but from folio 21 onwards his letters are much smaller. This variance in the size of script means that the number of lines per page also varies. This can be taken to indicate that there are multiple scribes or to the opinion that the single scribe was working at different periods of his life and bound the sections together.
Recently, the author of three of the religious poems has been identified as Master John of St. David's, who was there between 1148 and 1176. In the seventeenth century, it was at Hengwrt from where it went to Peniarth and eventually to the National Library of Wales in 1909 (MS Peniarth 1).
There are three poems definitely relating to the Arthurian legend, 'Yr Afallennau', 'Yr Oianau' and 'Ymddiddan Myrddin a Thaliesin'; a fourth poem 'Y Bedwenni' is also thought to be relevant. However, it is only in the 'Ymddiddan' that Myrddin is named, although the place-name Caerfyrddin occurs once in 'Yr Oianau'. In the first two poems, we find an unnamed man living alone in a forest addressing an apple tree in the former poem and a pig in the latter, and prophesies that were taken to foretell the success or failure of the Welsh in their battles with the Normans. The lines containing these prophecies were obviously composed after the events they purport to foretell and must be regarded as additions to a nucleus of lines composed much earlier, probably in the ninth or tenth century. These poems refer to a legend concerning Myrddin who goes mad during the Battle of Arderydd fought c. 573CE where Gwenddolau, his king, is killed. Some scholars place the battle with Rhydderch Hael due to the mention of Myrddin fearing capture by him; but John Morris and others believe that the other combatant group was Peredur. It was Geoffrey of Monmouth in his History of the Kings of Britain (1136) who first gave this prophet of Welsh tradition the name of Myrddin and connected him with the town of Carmarthen (Caerfyrddin). In 'Ymddiddan Myrddin a Thaliesin', we find Geoffrey's Myrddin in conversation with the poet Taliesin, who was also credited with prophetic powers.
