Daniel Rhodes – Adversary (New English Library, 1989)

Blurb:
Daniel Rhodes presents us with something more than a horror novel, skilfully blending centuries-old black magic into a modern setting.
In NEXT, AFTER LUCIFER, the undead spirit of Guilhelm de Courdeval, a medieval French Templar rumoured to have sold his soul to Satan, rose from centuries of imprisonment to possess an American college professor who probed too deeply into the legend of Courdeval’s occult powers and evil deeds.
Now, in ADVERSARY Courdeval resurfaces in San Francisco and resumes his career in the service of the Adversary, the enemy of mankind. Posing as a spiritual teacher, he extends his influence over four young Americans who all want something just beyond their reach. Lawyer Nicole Patrick is attractive and single – but the man she loves treats her like a sister. Spencer Epps has a sexy mistress and a rich father-in-law – but he’s been dipping into the till to support his illicit love life and cocaine habit, and things are closing in on him. Charity Haverill wants revenge on a lover who jilted her. And Robbie Kinsella is hungry for knowledge of the occult.
By offering seemingly harmless fulfilment of these desires, Courdeval elaborately seduces his “students” into joining him in the service of evil — until they find out, too late, the price they must pay for what they have received.
Meanwhile, Courdeval plays a sinister cat and mouse game with the only people who know the truth of his origin: the French priest Etien Boudrie and the housewife Melusine Devarre, gifted with second sight.
ADVERSARY is a chilling tale of desperation, revenge, and black magic.