Sagacious Blade. Sagacious, #4

The final book in the Phoenix Feather and Sagacious series, Sagacious Blade brings the story full circle. It would be difficult to find two boys more unlike. Friendly, animal-loving Ki Mek, youngest of a sprawling family of gallant wanderers, was born with Essence talent-much to the dismay of the Kis, who only believe in the skills of their blades. Je Tai, the scholarly, reclusive only son of a subsidiary branch of the lofty Ji family, is on his way back to the school where he is studying divination.
The two are swept overboard in a terrible storm, and end up made slaves by the western empire, age-old enemies of the Thousand Islands Empire. They escape into adventures martial and magical until they and those they love are swept up in a war spanning two empires. With the aid of Sagacious Blade, both will learn the costs of power.