The land of leys

A psychiatrist who by chance guilds his house on a barrow, an ancient burial ground, begins to dabble in black magic and to use it, in combination with hallucinogenic drugs, to blackmail rich and credulous old ladies. His manservant, who is a true practitioner of the black art and a Magister of the First Degree, turns the doctor's play acting into the real thing, and, at the story's end, manages to conjure up the very flames of Hell.