The Demon King's Assasin by Lily Raine
Anastasia’s cousin Jo, has warned her to keep away from their new accountant. Arkin is infatuated with Anastasia, but it isn’t until he asks for her help she comes to realize why. But when a deadly assassin learns of Arkin's plans to escape Destria, he turns to Anastasia.
A dagger from the other world, creatures who are neither human nor animal, and a web comic detailing future events are all Anastasia can rely on. And when the assassin’s interests turn to her beloved cousin, Mason, where is the line between who should live and who should die?