The Broken Eye by Brent Weeks

Book 3 As the old gods awaken and satrapies splinter, the Chromerie races to find the only man who can still end a civil war before it engulfs the known world. But Gavin Guile has been captured by an old enemy and enslaved on a pirate galley. Worse still, Gavin has lost more powers than his powers as Prism- he can't use magic at all.