Kingdom of the Wicked - Derek Landy

Magic is a disease. Across the land, normal people are suddenly developing wild and unstable powers. Infected by a rare strain of magic, they are unwittingly endangering their own lives and the lives of the people around them. Terrified and confused, their only hope lies with the Sanctuary. Skulduggery Pleasant and Valkyrie Cain are needed now more than ever. And then there's the small matter of Kitana. A normal teenage girl who, along with her normal teenage friends, becomes infected. Becomes powerful. Becomes corrupted. Wielding the magic of gods, they're set to tear the city apart unless someone stands up against them. Looks like it's going to be another one of those days...