Seasons of War by Derek Landy

The thirteenth thrilling novel in the internationally bestselling Skulduggery Pleasant series, SEASONS OF WAR will test the Skeleton Detective and Valkyrie like never before... War is coming. To avert catastrophe, Skulduggery and Valkyrie are sent on a secret mission that takes them away from everything they know, to a forsaken land of magic and grim, unrelenting terror. It is here that Valkyrie will have to fight the hardest - not only against the enemies who want her dead, but also against her own self-destructive impulses. It's only by crawling through darkness that she'll be able to once again stand in the light...