The Violet Keystone (The Seventh Tower #6) by Garth Nix

Bestselling author Garth Nix's amazing Seventh Tower series is back--now with a great new look!

Tal and Milla are only one step away from death. Evil is only one step away from triumph. An old war is made new again. The ultimate power is up for grabs.
As shadows rage, ancient spells return, illusion reigns and corruption threatens. The search for hope is the bravest quest.
The Seventh Tower is the key to it all.