Yashakiden - The Demon Princess (Novel)

After a devastating earthquake that left most of Japan in ruins, the district ward of Shinjuku in Tokyo has opened up a doorway into another dimension—becoming the halfway-house suspended between the mortal world and the supernatural. Setsura Aki, undercover P.I., is considered the best "man hunter" in the business. He allies himself with friend and rival, Doctor Mephisto—a wizard of physical and spiritual surgery. Together, they battle the evil forces that overrun the city. Coming from the deepest and darkest parts of China, four immortal demons, led by the Princess of vampires, have surmounted four thousand years of space and time to seize control of Shinjuku and make it their own. It is up to Setsura and Mephisto to uncover their new formidable foes’dark plans for the city. Can the duo stop them before they plunge the living populace into a new level of fear and slavery none can escape? (taken from mangaupdates)