Runelight — Joanne Harris

The squabbling Norse gods and goddesses of Runemarks are back! And there's a feisty new heroine on the scene: Maggie, a girl the same age as Maddy but brought up a world apart - literally, in World's End, the focus of the Order in which Maddy was raised. Now the Order is destroyed, Chaos is filling the vacuum left behind... and is breaching the everyday world.

A chilling prophecy from the Oracle. A conflict between two girls. And with just twelve days to stave off the Apocalypse, carnage is about to be unleashed....