The Stone Key: The Obernewtyn Chronicles Volume 5

There was a great crash and wood splintered . . . I had a brief glimpse of a group of Herder priests, bald and robed, peering at me, and then the sundered remnants of the locker door were torn aside and a rough hand reached in to haul me out by the hair. A Hedra captain stared into my face with eyes that burned with a fanatical fire above a thin nose and a lipless slash of a mouth. . .
'You will die in great pain and very slowly, mutant,' said the Hedra master.
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. Capture 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 weaponmachines 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 . . .