Last stand of dead men
(Book)

Originally published in Great Britain by HarperCollins Children's Books in 2013.

War has finally come. But it's not a war between good and evil, or light and dark--it's a war between Sanctuaries. For too long, the Irish Sanctuary has teetered on the brink of world-ending disaster, and the other Sanctuaries around the world have had enough. Allies turn to enemies, friends turn to foes, and Skulduggery and Valkyrie must team up with the rest of the Dead Men if they're going to have any chance at all of maintaining the balance of power and getting to the root of a vast conspiracy that has been years in the making. But while this war is only beginning, another war rages within Valkyrie herself. Her own dark side, the insanely powerful being known as Darquesse, is on the verge of rising to the surface. And if Valkyrie slips, even for a moment, then Darquesse will burn the world and everyone in it.

In English.