Broken Mirrors (Marla Mason Series #5)

Sorcerer Marla Mason has never been afraid of a fight, but she's about to face her greatest challenge: herself. Or, at least, an alternate-universe version of herself, a woman who succumbed to the evil power of a cursed artifact and become a monstrous villain. Now this dark doppelganger is loose in Marla's world... and she has an agenda even more terrible than Marla can imagine.