Geoffrey of Monmouth

Author of <a rel="nofollow" class="external text" href="https://en.wikipedia.org/wiki/Historia_Regum_Britanniae">The History of the Kings of Britain</a>, a work thought to be a historical account of British history, its kings, and most famously popularised the mythos of King Arthur. It is now considered false history with one 12th-century historian writing "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."

The section on Wikipedia about Geoffrey's contemporaries had this hilarious bit:

Giraldus Cambrensis recounts the experience of a man possessed by demons: "If the evil spirits oppressed him too much, the Gospel of St John was placed on his bosom, when, like birds, they immediately vanished; but when the book was removed, and the History of the Britons by 'Geoffrey Arthur' [as Geoffrey named himself] was substituted in its place, they instantly reappeared in greater numbers, and remained a longer time than usual on his body and the book."