Dragonfly Falling by Adrian Tchaikovsky.
Second book in the enthralling “Shadows of the Apt” series of epic fantasies.
Two young companions, Totho and Salma, arrive at Tark to spy on the menacing Wasp army, but are there mistakenly apprehended as enemy agents. By the time they are freed, the city is already under siege. Over in the imperial capital the young emperor, Alvdan, is becoming captivated by a remarkable slave, the vampiric Uctebri, who claims he knows of magic that can grant eternal life. In Collegium, meanwhile, Stenwold is still trying to persuade the city magnates to take seriously the Wasp Empire’s imminent threat to their survival.