Skulduggery Pleasant: End Of The World.

End Of The World is an exclusive Skulduggery Pleasant book that was released on World Book Tag 2012. Its not a proper book in the series but a short story of 111 pages and a note from Derek Landy.

The blurb reads:

Ryan is an ordinary boy living in an ordinary world.
Or so he thought.
Pursued Von a gang of insane sorcerers who want to destroy the planet, Ryans only chance for survival rests with skeleton-detective Skulduggery Pleasant and his teenage partner Valkyrie Cain. They have one Tag to save Ryan. One Tag to save the world. And the clock is ticking......