The Stone Key: The Obernewtyn Chronicles Volume 5 by Isobelle Carmody
When Farseeker Guildmistress Elspeth Gordie sets out from Obernewtyn to travel to Sutrium at the end of Wintertime, she quickly learns that not everyone welcomes the changes brought about by the rebellion.
Captured by an old and vicious enemy, she is drawn into the heart of the Herder Faction, where she learns of a terrible plot to destroy the west coast.
To stop it, Elspeth must risk everything, knowing that if she dies, she will never complete her quest to find the weapon machines that destroyed the Beforetime.
But if she succeeds, her journey will lead her to the last of the signs left for her by the seer Kasanda . . .