Mortal Coil ( Skulduggery Pleasant series )

In this book, the magical world is completely thrown into chaos. Valkyrie discovers a terrifying secret—that she is prophesied to destroy the world—and goes on a personal quest to prevent her dark destiny from coming to pass. Meanwhile, the Remnants (thousands of twisted, malevolent souls capable of possessing anyone) break free from the Midnight Hotel, plunging Skulduggery, Valkyrie, and the rest of the gang into a terrifying situation where they cannot trust anyone.