The Broken Eye: Book 3 of Lightbringer by Brent Weeks
As the old gods awaken and satrapies splinter, the Chromeria races to find the only man who might 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 than his powers as Prism - he can't use magic at all. Without the protection of his father, Kip Guile will face a master of shadows as his grandfather moves to choose a new Prism and put himself in power. With Teia and Karris, Kip will have to use all his wits to survive a secret war between noble houses, religious factions, rebels, and an ascendant order of hidden assassins called The Broken Eye. With over two million books in print, Brent Weeks has become one of the fastest selling new fantasy authors of all time.