The Kingdoms Of Dust - The Necromancer Chronicles: Book Three

With her master dead and her oaths foresworn, necromancer and spy Isyllt Iskaldur finds herself in exile. Hounded by assassins, she seeks asylum in Assar, the empire she so recently worked to undermine. There, warlords threaten the Empire's fragile peace, and the empress is beset by enemies within the court. Even worse, darkness stirs in the deep desert. Ancient spirits are waking that could destroy Assar faster than any army.
Isyllt must travel into the heart of the desert to lay the darkness there to rest once more. But first she must stop an order of mages that will do anything to keep the Empire safe - even raze it to dust.