Geoffrey of Monmouth
Geoffrey of Monmouth (c.1100 - c.1155) was a clergyman and one of the major figures in the development of British history. He was born in Wales, possibly of Breton ancestory. He studied at Oxford University, where he met Walter, who was Archdeacon of Oxford. On 21 February 1152 Archbishop Theobald consecrated Geoffrey as bishop of St Asaph, having ordained him a priest 10 days before. "There is no evidence that he ever visited his see" writes Lewis Thorpe, "and indeed the wars of Owain Gwynedd make this most unlikely." Geoffrey attested about six different charters between the years 1129 and 1151; the date of his date is recorded in the Welsh Chronicles.
Geoffrey of Monmouth wrote several works of interest. The earliest one to appear was Prophetiae Merlini ("The Prophecies of Merlin"), which he wrote at some point before 1135. Geoffrey presented a series of apocalyptic narratives as the work of the earlier Merlin who, until Geoffrey's book came out, was known as "Myrddin". The first work about this legendary prophet in a language other than Welsh, it was widely read — and believed — much as the prophecies of Nostradamus were centuries later; John Jay Parry and Robert Caldwell note that the Prophetiae Merlini "were taken most seriously, even by the learned and worldly wise, in many nations", and list examples of this credulity as late as 1445.
Next was Historia Regum Britanniae (History of the Kings of Britain), the work best known to modern readers. It claims to relate the history of Britain down to the 7th century, but was in fact a work of fiction. (John Morris in The Age of Arthur calls it a deliberate spoof.) It includes numerous legends, including the claim that Trojan hero Aeneas was the ancestor of the first line of British kings. It is one of the first texts to mention King Arthur (having him subdue most of Europe, no less) and is the first surviving text to mention King Lear (as King Leir).
Lastly, Geoffrey wrote the Vita Merlini ("The Life of Merlin") at some point between 1149 and 1151. This is Geoffrey's own retelling of the earlier Myrddin legend from Welsh tradition.
All of these books were written in Latin, as were most learned works of the medieval period.
Most modern scholars acknowledge Geoffrey's work to be largely fiction containing little trustworthy historical fact -- although many are tempted to agree with William of Newburgh, who wrote around 1190 that "it is quite clear that everything this man wrote about Arthur and his successors, or indeed about his predecessors from Vortigern onwards, was made up, partly by himself and partly by others, either from an inordinate love of lying, or for the sake of pleasing the Britons." Further, his structuring and reshaping of the Merlin and Arthur myths has had a huge influence in the perception of those figures ever since: he may be viewed as the major establisher of the Arthurian canon.