The Sword of Unda — Meg Hendry

Things are different this time. The cultists of Unda, a group calling themselves Tempest, are searching for something in the country of Mendus. They are also gathering allies for Unda, the Guardian of Water. She has had many years to make her plans, to prepare, years where no one rose to stop her. No one knew she needed stopping. Kian, Dimitri, Helen, and Vetus are drawn into the fight against Tempest, but according the Seer of Mendus none of this was meant to happen. The Sword of Unda is a novella.