Who was the Welsh cleric who lived from ca. 1100 to 1155 and wrote "The Prophecies of Merlin", the poetic "Life of Merlin", and the "History of the Kings of Britain", which contains the earliest full account of King Arthur's life as well as some of the life of Arthur's father Uther Pendragon?

His name was Geoffrey of Monmouth.

Explanation:

Geoffrey of Monmouth was a cleric born between 1095 and 1100 who is famous to this day for his influence over the Arthurian myths. Much of his life cannot be accounted for since information is scarce. We do not know precisely where he was born; some sources say he was Welsh, others say he was British. The exact year when he was born is also controversial.

Geoffrey was the author of the "History of the Kings of Britain", or Historia Regum Britanniae, which was translated into several languages. Nowadays, this work is considered unreliable. But Geoffrey's earliest work was probably the Prophecies of Merlin which, as its name reveals, contains a number of prophecies attributed to the wizard Merlin. Some say the character Merlin was created by Geoffrey himself, but Geoffrey claimed to have based him in older Brittonic traditions.