Necropolis — Anthony Horowitz

As the fourth novel in the spellbinding Gatekeepers series begins, the world is under the greatest threat it's ever known. The evil corporation Nightrise has amassed an immense amount of power . . . and the devastating force of the Old Ones is about to be unleashed around the globe. To stop this from happening, Matt and three of the Gatekeepers head to Hong Kong--not just the modern city of skyscrapers and wealth, but the secretive underworld beneath. In Hong Kong they will meet the final Gatekeeper, a girl named Scarlet, whose fate is inextricably joined to their own....