Ghost in the Mask. The Ghosts, #8

Caina Amalas is a nightfighter of the Ghosts, the spies and assassins of the Emperor of Nighmar, and through cunning and valor she has cast down both proud lords and mighty sorcerers. But a power darker than any she has ever faced is awakening. When a mad assassin armed with a sorcerous blade rampages through the Emperor's capital, Caina must track the weapon to its origin, a ruined city blighted by the folly of sorcerers, a city that holds weapons far more potent than mere blades.
Weapons that can resurrect an ancient empire of dark sorcery to enslave the world anew...