Death Bringer: Skulduggery Pleasant #6 by Derek Landy

Meet Skulduggery Pleasant: detective, sorcerer, warrior. Oh yes. And dead. The Necromancers no longer need Valkyrie to be their Death Bringer, and that’s a Good Thing. There’s just one catch. There’s a reason the Necromancers don’t need her any more – because they’ve found their Death Bringer already, the person who will dissolve the doors between life and death. And that’s a very, very Bad Thing…  

Derek Landy