The Devil Rides Out by Dennis Wheatley
The aristocratic Duke de Richleau faces new, sinister challenges in this macabre tale of the dark arts. When his good friend Simon Aron's naive curiosity is tested, the Duke, along with his ever-patient friends Rex Van Ryn, and Richard Eaton, must intricately plot a means of both physical and spiritual rescue. But with Van Ryn's affections for a beautiful woman caught in the web of Satanists, and Eaton's ongoing scepticism, they all risk being brought to the verge of madness through dabbling with the powers of evil. From London to the West Country, the slums of Paris to a Christian monastery, the action of this powerful occult thriller moves with fantastic, compelling force.