Rise of Empire - Michael J. Sullivan

(Riyria Revelations book 3-4)

The birth of the Nyphron Empire has brought war to Melengar. To save her kingdom, Princess Arista runs a desperate gamble when she defies her brother and hires Royce and Hadrian to perform a dangerous mission behind the enemy's lines.

As the power of the Nyphron Empire grows, so does Royce's suspicion that the wizard Esrahaddon is using the thieves as pawns in his own shadowy struggle for power. To find the truth, he must unravel the secret of Hadrian's past. What he discovers leads the thieves to the ends of the world on a journey amid treachery and betrayals, forcing Hadrian to face a past he hoped never to see again.