The Assassin's Way By JM Kearl

Never go out at night. Lock the doors. Ignore the screams.
Those are the rules Aesira lives by to survive the monsters that stalk her village. But everything changes when it’s revealed she’s been born with the power to fight the vampires she’s spent her life fearing.
Aesira is thrust into the elite and secretive League of Assassins, where mercy is weakness and fear is forbidden. Now her survival depends on her ruthless trainer, Vander – the most dangerous assassin of them all. And the only thing more lethal than his blade is the desire he awakens in her.
Because the greatest threat in Nighthaven might not be the monsters lurking in the dark…
It might be him.