The Pirates of Pacta Servanda (The Pillars of Reality #4)

Master Mechanic Mari and Mage Alain have survived every attempt to stop them, but their enemies are determined to kill Mari, the only one who can save her world from a storm of destruction. As armies begin to gather and cities seethe with tension, Mari, Alain, and their friends must prepare to confront the storm in the place it first appeared: the broken kingdom of Tiae.

The dangers facing them demand perilous raids, tough battles, and more than a little piracy. Beating the Mechanics Guild and the Mage Guild that have controlled the world of Dematr for centuries will require an unprecedented alliance under Mari's command of rebellious Mechanics, Mages, and common folk who know that together they can change the world, but separately they will all lose.