Maohden Vol 1 by Hideyuki Kikuchi

Demon City Shinjuku, home to monsters and magicians alike. The warlock Gento Roran has broken his fifteen-year slumber and seeks dominion over the city. Now resurrected from his accursed sarcophagus, he is accompanied by the Hyota, the "small superman."
A secret seal has proclaimed that "Those who control Demon City will control the world." And so the stage is set for a fateful battle between Gento and Setsura Aki.
Meanwhile, Doctor Mephisto, the Demon Physician, is treating a curious new patient, a girl named Mayumi. Any man who has sex with her dies soon afterwards. This deadly cause and effect appears to be nothing more than coincidence at first. But only at first.
Assassination squads possessed of the most unusual ways of dealing death come after Setsura. One after the next, he's caught up in the fight of his life! What is this secret seal? And how is it connected to Mayumi?
Hideyuki Kikuchi has again unleashed his creative powers in a thrilling and hair-raising tale of the supernatural. The first volume of Maohden!