Merlin's origin

According to a tradition not found in Malory, but very likely familiar to Malory's readers, Merlin was engenderd, despite all precautions, by a fiend on a woman who had been trying to remain pure.

Conception of Merlin, from the prose Lancelot, c. 1494.

At her trial, the infant Merlin himself revealed who was his father and made a few other prophecies and revelations, at least one of them rather embarrassing to the judges. Some say the father of Merlin was a demon of air (incubus) and he was raised in a nunnery. Robert says the devils of Hell had determined to set on earth an evil being to counterbalance the good introduced by Jesus Christ. Happily, the child was promptly baptized so he was not evil!

Even in Malory's account, Merlin is occasionally called a devil's son, and now and then a medieval romancer throws in a comment to the effect that, although usually regarded as beneficent, Merlin is really evil. In all fairness, however, I must point out a remark in Vulgate II that, while Merlin owed his knowledge of the past to the Devil, he owed his knowledge of the future to the Lord. Again, Vulgate II tells us that Merlin never laid his hands upon anyone, though with his power, he hardly needed to be physical! Vulgate II contains a statement that he was "treacherous and disloyal by nature" and mentions a stone at which he slew two other enchanters.

Merlin's mother was called Aldan in Welsh tradition, Optima in French romance and Marinaia in Pieri's Storia di Merlino (fourteenth century). The Elizabethan play The Birth of Merlin - which may have been partially authored by Shakespeare - calls her Joan Goto-'t. That he had no father does not seem to be a feature of Welsh tradition. He was also said to be the son of Morgan Frych who, some claimed, had been a prince of Gwynedd.