Mistress of Lies by K.M. Enright

The daughter of a powerful but disgraced Blood Worker, Shan LeClaire has spent her entire life perfecting her blood magic, building her network of spies, and gathering every scrap of power she could. Now, to protect her brother, she assassinates their father and takes her place at the head of the family. And that is only the start of her revenge.

Samuel Hutchinson is a bastard with a terrible gift. When he stumbles upon the first victim of a magical serial killer, he’s drawn into the world of magic and intrigue he’s worked so hard to avoid – and is pulled deeply into the ravenous and bloodthirsty court of the vampire king.

Tasked by the Eternal King to discover the identity of the killer cutting a bloody swath through the city, Samuel, Shan and mysterious Royal Blood Worker Isaac find themselves growing ever closer to each other. But Shan’s plans are treacherous, and as she lures Samuel into her complicated web of desire, treason and vengeance, he must decide if the good of their nation is worth the cost of his soul.