The Ambassador's Mission by Trudi Canavan

Sonea, once the despised commoner in the Magicians' Guild, is now a Black Magician of Kyralia. Though she is now part of the establishment, she comes to find that the past is not so easily left behind...
Lorkin, Sonea's rebellious son, has volunteered to join the formidable Lord Dannyl in his new post as Guild Ambassador to Sachaka - a land ruled by cruel, slave-owning black magicians. There is suspicion that some of the Sachakans still harbor dreams of conquest, so when word comes that Lorkin has gone missing, Sonea is desperate to find him. However, Guild law states that if a Black Magician leaves the city he or she will be exiled forever.
As she fears for her missing son, Sonea dedicates herself to helping her old friend Cery. For some time, there has been fear and paranoia on the streets of Imardin. Leading thieves have been dying under irregular circumstances. The need to discover who has been picking off the leading thieves of the city is now a very personal crusade - one that Sonea must aid, for the killer appears to be using magic.
Either a member of the Guild is leading a double life as a hired killer, or there is - once again - a rogue magician on the streets of Imardin . . .

This book is part of the "The Traitor Spy Trilogy" series.
Here are some other books from this series: