For years, Valkyrie Cain has struggled to keep her loved ones safe from harm, plunging into battle time and time again by Skulduggery Pleasants side, and always emerging triumphant. But now the very thing that Valkyrie fights for is in danger, as a ruthless killer snatches her little sister in order to lure Valkyrie into a final confrontation. With Skulduggery racing to catch up and young sorcerer Omen scrambling along behind, Valkyrie only has twelve hours to find Alice before its too late.

On a desperate journey to recover her sister's lost soul, Valkyrie Cain goes up against the High Sanctuary itself, and there's nothing Skulduggery Pleasant can do to stop her. With Abyssinia's grand plan about to kick off in a night of magic, terror, and bloodshed, it falls to Omen Darkly to save the lives of thousands of innocent people. 

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.

To prevent this, Skulduggery Pleasant and Valkyrie Cain have one last - terrible - option: the assassination of Damocles Creed. With protests stirring in the magical city of Roarhaven, with riots and revolutions on the horizon, Valkyrie must decide who she wants to be: the hero who risks everything for a noble ideal, or the killer who sacrifices her own soul for the fate of humanity. The decision must be made, and time is running out.