The Oath Breaker (Blood Vier #0.5)

Before Taryn and Vladimir and the great war that shook the continent, the Kavari Michael Gallows sought a weapon to end his own war, and Evelyn Illdalore was destined to follow in the steps of her father but always dreamed of venturing beyond her city's walls.

A chance encounter entwined their fates, testing their loyalties and desires and awakening hopes of a different future. But the cost of love demanded more than they could ever sacrifice, and to break the binds of oaths and duty would mean forsaking everything.

The Oath Breaker explores the origin story of Taryn's parents, and while a prequel, it should only be read after finishing all four books in the Blood Vier quartet.