Necropolis - Anthony Horowitz

This book is by Anthony Horowitz. The 4th book and also the second last book of the Power of Five series. In this book, Matthew Freeman, the leader of the gatekeepers, finally finds the 5th gatekeeper who is the last gatekeeper, Scarlett Adams. Her father works for a company called Nightrise, which turns out to be an evil organization that is controlled by the Old Ones - the forces of evil. The Old Ones make Scarlett's father, Paul Adams, bring her to Hong Kong, where Nightrise is, to capture her because they know she is one of the 5 gatekeepers. In return, they said that they would pay Paul lots of money. Paul lures Scarlett to Hong Kong but when he asks for the money that he was told to be rewarded with, the Old Ones kill him instead.
The Old Ones plan to turn Hong Kong into a Necropolis - A city of the dead. Scarlett is therefore trapped in that dying city, where the air is poisoned, and whats worse is that there can be NO escape...
The only way to defeat the Old Ones now is for the 5 gatekeepers to get together. But with Scarlett trapped in Hong Kong, that will be impossible.
Of course the Old Ones have been tricked and defeated before, but this time they have a plan and they are in complete control.