The Vacant Throne (Throne of Amenkor #3) by Joshua Palmatier

Ruled by the Mistress of the Skewed Throne, the city of Amenkor has just survived a devastating invasion, paying a terrible price both in the loss of lives and destruction in the city itself. Yet perhaps the most crucial loss is the throne of Amenkor-the true seat of power in every sense-now totally drained of the magic, knowledge, life force, and memories of previous rulers. The city's only hope lies with its sometime ally, the city of Venitte, home to the throne that is twin in power to Amenkor's, the two created at the same time by a magic that no one can now duplicate.