A Son of Perdition by Fergus Hume

Enistor returned to Tremore in a black silent rage, wishing heartily that he could find some one with whom to discuss the position in which he found himself. But there was no one, as the mere statement that Narvaez had taken possession of Hardwick's body would be scouted by the most credulous. There were some things which could not possibly be believed, and this was one of them. The present generation was too material to entertain, for a single moment, so wild an idea, and Enistor knew -- as his master had warned him -- that he ran a very good chance of being locked up as a lunatic, if he even hinted at the astounding truth. Thus the Squire's dabbling in unholy matters had isolated him from his fellow-creatures, and, at the moment, he felt the deprivation keenly. There was nothing he could do but shut himself up in his library and think over his position.

It was not a pleasant one. By his own acts in this and previous lives he had committed himself to bondage, and was treated like a slave who had no rights of his own. What a fool he had been to doubt Narvaez' power, since again and again he had received proofs of it. On the evil hill beside the Druidical altar he had seen with other than physical eyes the terrible elemental creatures which Don Pablo could evoke, and whom he controlled by his powerful will. Any doctor would have told him that it was a case of mere hallucination brought about by the hypnotic suggestion of certain weird ceremonies. But the Squire knew better. There were ways of entering the invisible world which interpenetrates the visible sphere, and Narvaez, by centuries of study and training, had learned how to open the door. He had flung it wide to his pupil, but it could not be closed again, as Enistor did not know the necessary conjuration.