Nightfeeder

From the back cover of the Roc first edition: "The lure of magic, the lust for blood - Galen Sword was one of the richest men in the world, a carefree playboy. Then came the accident that changed his life and introduced him to a brother who brought Galen back from the dead and awakened memories of a long-forgotten childhood. For Galen, totally human and totally lacking in magical talents, had been born in the First World, a realm of elementals, shapechangers and vampires. Now, determined to return to the First World, Galen would use any help he could get - from the knowledge and skills of two brilliant physicists to the magical aid of a halfling who prowled the haunts of human New York City to find the passageways to the Shadow Realms and beyond. But his search made Galen the focal point between warring clans... and his very survival in the battle rested with that most dreaded of legendary beings, an adept whose powers could only be fueled by the blood of the living - the Nightfeeder"