The Heart of Stars: Book Three of Rhiannon's Ride

With the leading clan bitterly split and the court torn apart, only fierce, half-human Rhiannon and her winged mare have any chance of rescuing Princess Olwynne and her twin brother. But what duty requires, the heart may regret.
 
An evil necromancer has kidnapped the royal twins, plotting to spill their blood—for revenge, and to restore his own family to life. Chaos reigns in the capital, and Rhiannon finds herself burdened with a heavy charge: to save the one woman she hates above all others. It will not be easy to forgive or forget that Olwynne attempted to steal Rhiannon’s true love, the handsome apprentice-witch Lewen—especially when it seems that Olwynne would still be her rival for his affections. And as the necromancer’s intrigues grow more desperate, Rhiannon fears that saving the princess could mean risking everything—Lewen, her mare, her freedom, even her own life.