The Fiend Queen — Barbara Ann Wright

Within the walls of her palace, Princess Katya's best friend lies at her feet, close to death. Her pyradisté is overwhelmed by some mysterious power, and her former lady-in-waiting has stabbed her in the back. Wounded and nearly alone, Katya must find a way to sabotage the magic of her Fiendish uncle Roland, or those who fight for the capitol will be overwhelmed by hypnotized guards and Fiend-filled corpses.

Starbride's pain is nearly overwhelming. The agony inside her only lessens when she satisfies a strange new desire to hurt those around her. She may hold the key to banishing Fiendish power from Farraday, but only by using it herself. Together, Katya and Starbride must make a final desperate push to take back the kingdom, but even if they survive, can the strength of their love keep them from madness? After all, fighting evil with evil has its consequences.