Dead or alive (Skulduggery Pleasant Book 14), Derek Landy

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

In a matter of days, the world will change. Billions of lives will be wiped away in a final, desperate search for the Child of the Faceless Ones – she who is destined to bring about the return of humankind's ancient overlords. 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.