The Ambassador's Mission by Trudi Canavan

As the son of the late High Lord Akkarin, saviour of the city, and Sonea, the former street urchin turned Black Magician, Lorkin has a legacy of heroism and adventure to live up to. So when Lord Dannyl takes the position of Guild Ambassador to Sachaka, Lorkin volunteers to be his assistant in the hopes of making his mark on the world. When news comes that Lorkin is in danger, the law that forbids Black Magicians leaving the city forces Sonea to trust that Dannyl will save him. Besides, her old friend Cery needs her as never before: someone has been assassinating Thieves, and when his family is targeted he finds evidence that this Thief Hunter uses magic. Either a member of the Guild is hunting down the Thieves one by one, or there is - once again - a rogue magician on the streets of Imardin. And this one has full control of their powers - and is willing to kill with them.