The Ambassador's Mission (The Traitor Spy Trilogy, 1)

Half the Thieves in the city are dead and nothing can account for the randomness and the inventiveness of each death.

Sonea, a Black Magician of Kyralia, knows that she is needed to help hunt down the rogue magician killing the Thieves. But Sonea has problems of her own. Her son is now the assistant to the new Guild Ambassador of Sachaka and will be in deadly danger when he sets foot on their ancient enemy's soil. As a Black Magician, however, her every action is watched - and monitored. Any attempt to leave the city will result in her exile, and lose her any chance of helping her friends - or her son.