Night of the Demons

American psychologist John Holden arrives in London to participate in a symposium aiming to expose witchcraft and devil-worship as a fraud, with attention centered on a cult run by a Julian Karswell. Debarking his plane, Holden learns that the symposium leader, Henry Harrington, has suddenly died and he is now in charge. What only the audience knows is that Harrington was clearly killed by a demon