Crownbreaker Spellslinger Series, Book 6

Once an outlaw spellslinger, Kellen Argos has made a life for himself as the Daroman Queen's protector. A little magic and a handful of tricks are all it takes to deal with the constant threats to her reign. But when rumors of an empire-shattering war begin to stir, Kellen is asked to commit an unimaginable act to protect his queen. Inside enemy territory, he quickly realizes something is amiss. Someone is playing a dangerous game. And to discover their secrets, Kellen will have to challenge the greatest spellcaster who's ever lived.