Shadows After Dark by Ouida Crozier

Wings of death are spreading over the world of Kornagy and Kyril's mission on Earth is to find the cause. While here, she meets the beautiful but lonely Kathryn, who has been yearning for a deep and enduring love with just such a woman as Kryil. But to her horror, Kathryn learns that her darkly exotic new lover has been sent to Earth to save her own dying vampire world.