We shall never know how much of Geoffrey of Monmouth's Latin History of the Britons, which was largely responsible for the creation of the Arthurian legend, was based on fact, how much was mythical, and how much was pure invention; but even in his own day it was looked upon with suspicion. The narrator of the following sardonic anecdote was Giraldus Cambrensis (1146?-1220?), a turbulent ecclesiastic and a man accustomed to speaking his mind.
It is worthy of observation that there lived in the City of Legions in our time a Welshman called Melerius, who, under the following circumstances, acquired the knowledge of future and occult events. Having on a certain night, namely that of Palm Sunday, met a damsel whom he had long loved, in a pleasant and convenient place, while he was indulging in her embraces, suddenly, instead of a beautiful girl, he found in his arms a hairy, rough, and hideous creature, the sight of which deprived him of his senses, and he became mad. After remaining for many years in this condition, he was restored to health in the Church of St. David's through the merits of the saints. But having always an extraordinary familiarity with unclean spirits by seeing them, talking with them, and calling each by his proper name, he was enabled through their assistance to foretell future events. He was indeed often deceived (as they are) with respect to circumstances at a great distance of time and place, but was less mistaken in affairs which were likely to happen nearer, or within the space of a year. The spirits appeared to him usually on foot, equipped as hunters, with horns suspended from their necks, and truly as hunters not of animals, but of souls. He particularly met them near monasteries and monastic cells; for where rebellion exists, there is the greatest need of armies and strength. He knew when anyone spoke falsely in his presence, for he saw the Devil, as it were, leaping and exulting upon the tongue of the liar. If he looked on a book faultily or falsely written, or containing a false passage, although wholly illiterate he would point out the place with his finger. Being questioned how he could gain such knowledge, he said he was directed by the demon's finger at the place. In the same manner, entering into the dormitory of a monastery, he indicated the bed of any monk not sincerely devoted to religion. He said that the spirit of gluttony and surfeit was in every respect sordid; but that the spirit of luxury and lust was more beautiful than others in appearance, though in fact most foul. 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 [Geoffrey of Monmouth] was substituted in its place, they instantly reappeared in greater numbers, and remained a longer time than usual on his body and on the book.
From Giraldus Cambrensis, The Historical Works, translated Sir Richard Colt Hoare, Bart. (1863), pp. 374-5 (from The Itinerary of Wales).