Historia Regum Britanniae

The full, ancient text: Historia Regum Britanniae. Historia regum Britanniae (or The History of the Kings of Britain) is a supposedly historical account written by Geoffrey of Monmouth in 1136. Though much of the text is largely considered fiction, it does pull from several ancient texts and true historical events/personas. It is notable for being the first, major blockbuster-like success of the Arthurian legends, bringing the character to widespread popularity for the first time. Many of our modern myths (and ancient ones) have drawn from this text.

This volume also contains the separate Vita Merlini (or the Life of Merlin) which was written later by Geoffrey and often added into subsequent editions of the book.