Necromancer — George Herman

A charismatic priest suddenly appears in Rome and claims to be both holy man and a master of black arts. He conducts "summonings" in the old Roman Coliseum where thousands of demons appear and call for the deaths of certain cardinals and eventually the Pope, Alexander VI-all are realized within days.
With help from forbidden books in monastery libraries, and the undercover investigations of street urchins known as Vavias, Leonardo uncovers the tricks used to create the demon illusions. At the request of Cesare Borgia and Cardinal della Rovere, Leonardo challenges the priest to a battle of magic, which exposes the tricks. The outcome puts Leonardo in danger of being entombed alive.