The castoffs. Volume 4, Byte back
(Book)

"Ursa is reunited with her mother at last--but her mom is the Priestess, leading a massive Surrogate army that threatens to swallow the town of Plumstead and then the world! She's got Charris and Trinh by her side, and a militia of mages and townsfolk at her back, but will that be enough? The Priestess's massive force is held at bay by a magical barrier--but with its caster trapped inside with the Surrogate, wasting away, this precarious standoff could give way at any time! These young mages may have learned to trust each other, their allies, and themselves, but this battle could define the fate of the world."--Provided by publisher.