Seasons of War (Skulduggery Pleasant)

Skulduggery Pleasant is dead. Valkyrie Cain is cool. Omen Darkly is neither. 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…