The Emperor's Blades by Brian Staveley
"The emperor of Annur is dead, slain by enemies unknown. His daughter and two sons, scattered across the world, do what they must to stay alive and unmask the assassins. But each of them also has a life-path on which their father set them, their destinies entangled with both ancient enemies and inscrutable gods.
Kaden, the heir to the Unhewn Throne, has spent eight years sequestered in a remote mountain monastery, learning the enigmatic discipline of monks devoted to the Blank God. Their rituals hold the key to an ancient power he must master before it’s too late.
An ocean away, Valyn endures the brutal training of the Kettral, elite soldiers who fly into battle on gigantic black hawks. But before he can set out to save Kaden, Valyn must survive one horrific final test.
At the heart of the empire, Minister Adare, elevated to her station by one of the emperor’s final acts, is determined to prove herself to her people. But Adare also believes she knows who murdered her father, and she will stop at nothing—and risk everything—to see that justice is meted out."