The Dying of The Light (Skulduggery Pleasant Book 9), Derek Landy

He’s dead. She’s deadly. The bad guys don’t stand a chance.

The ninth book in the bestselling Skulduggery Pleasant series. Following the loss of Valkyrie Cain, Skulduggery Pleasant must track down Darquesse before she turns the world into a charred, lifeless cinder – and so he draws together a team of soldiers, monster hunters, criminals and Valkyrie’s now murderous reflection. Not everyone gets out of this alive.