The Wind that Sweeps the Stars — Greg Keyes

A taut high fantasy as an assassin must destroy an empire from within, eliminating wizards, their demons, and even the emperor. For the lives -- for the very souls -- of her people, she must succeed within a single day, or her homeland will be destroyed.

ALONE AGAINST AN EMPIRE.
When Yash of Zeltah arrives in the fortress city of Honaq, she is greeted as a barbarian, a simple pawn. Her marriage to prince Chej has been arranged, they say, to avert war. Yet she knows the truth, for the armies already ravage the land. A skilled and deadly assassin, there is more to Yash than any might suspect. Before another day can pass, she must defeat the masters of the nine towers -- the plagues, magics, and monsters they control, the soldiers they command. Without raising an alarm, she must kill all who oppose her -- even the immortal emperor. The lives and souls of Zeltah, the people and the land upon which they live, all depend on it.