Diogenes Trilogy, Book 1.
Behind the gates of a fabulous Hamptons estate, FBI Special Agent Pendergast comes upon the carnage of a gruesome crime: one that recalls the legendary horrors that befall those who make a Faustian pact with the devil. Surrounded by the choking stench of brimstone, the smoldering remains of art critic Jeremy Grove are found in a locked, barricaded attic next to a hoofprint singed into the floorboards.
Unable to resist a case that defies all but supernatural logic, Pendergast reunites with police officers Vincent D'Agosta (Relic) and Laura Hayward (Reliquary) to search for a more earthly explanation. But their investigation soon takes them from the luxury estates of Long Island and penthouses of New York City to the crumbling, legend-shrouded castles of the Italian countryside, where thirty years ago four men conjured up something unspeakable...