36 righteous men / Steven Pressfield.

"New York homicide detectives pursue a serial killer in this apocalyptic thriller. When detectives James Manning and Covina 'Dewey' Duwai are called in to investigate a series of bizarre murders, they make a shocking discovery: the legend of the hidden righteous ones, the 36 who preserve the world from destruction, is no legend at all. They are real, and they are being murdered. As the bodies pile up and the world tilts into chaos, Manning and Dewey must protect the righteous ones from a ruthless killer able to beguile his victims and command them against their will. The detectives find their traditional arsenal of bullets and blades of little use against a foe who seems to anticipate their every move. Joining forces with a disgraced but brilliant rabbinical scholar and a renowned anthropologist--who's also the last of the righteous ones--Manning and Dewey set off on a perilous quest from New York to Gehenna to defeat a murderer who won't stop until he's killed everyone."--