S. M Stirling

Adrian Breze is more Shadowspawn than human, but he has turned from his own kind to fight for humanity. He battles the dark forces of the world, including those of his own blood ... including his sister twin anima.

"Near-purebred home nocturnus Adrian Breze and his human mate, Ellen, thought they had dealt with his twin sister, Adrienne. In fact, they thought she was dead. But she survived and now leads a faction of the Shadowspawn--the ancient, shape-shifting, blood-drinking breed who secretly control the world--that wants to leave just enough of the human race alive to satisfy their hunger and serve their needs. She has nothing but hatred and contempt for...