Darkborn, book 3: Shadowborn
Alison Sinclair2011

Magic dies with the mage, or so the Darkborn believe. That's why Lady Telmaine Hearne has been condemned to death for sorcery. She's escaped but is now bound with her mageborn allies for the Borders and war. Meanwhile, her husband, Balthasar, has learned of his family connection to the Shadowborn-and is fighting for survival and sanity as magic turns him against everything he holds dear.