Faustus Resurrectus by Thomas Morrissey

Unholy murder is just the beginning of the ritual...
When Donovan Graham, newly-graduated occult scholar, helps the NYPD investigate a man killed by scorpions in a midtown hotel, he learns the world is far stranger and deadlier than his studies ever suggested. Evidence forces his academic skepticism to give way to astonished belief that ancient evil exists, and the more he investigates, the higher it rises to overshadow the normality of his life. Can he save those he loves from its power?
In a Central Park overrun with madness, a suave sociopath seeks to achieve his darkest desires by tearing apart the world. Battling him through death and beyond, Donovan risks his soul to learn reality is flexible, and even the impossible can be had if a high enough price is paid...