Captain's Fury (Codex Alera #4) by Jim Butcher

After two years of bitter conflict with the hordes of invading Canim, Tavi of Calderon, now Captain of the First Aleran Legion, realizes that a peril far greater than the Canim exists--the mysterious threat that drove the savage Canim to flee their homeland.

Now, Tavi must find a way to overcome the centuries-old animosities between Aleran and Cane if an alliance is to be forged against their mutual enemy. And he must lead his legion in defiance of the law, against friend and foe--or no one will have a chance of survival . . .