Uncanny X-Men #30

The Warlock appears and summons Jean and Prof. Xavier to his subterranean cavern where his headquarters was an actual medieveal castle, intending to make Jean his queen. He revealed to them an apparatus he invented that was powered by his brain. It was suppose to turn the minds of normal mortals backwards- to the level of mankind at the time of King Arthur. They would no longer have the scientific weapons or skills to oppose the forces of the Warlock.The Warlock tells that he used to be Merlin the Magician, and onde had fight Thor.Angel had follown them, and Xavier manages to summon the rest of the X-Men to help destroy the Warlock and put him in a comatose state.