Fated (Fated, #1) by Sarah Alderson

What happens when you discover you aren't who you thought you were? And that the person you love is the person who will betray you? If your fate is already determined, can you fight it?

When Evie Tremain discovers that she’s the last in a long line of Demon slayers and that she’s being hunted by an elite band of assassins –Shapeshifters, Vampires and Mixen demons amongst them – she knows she can’t run. They’ll find her wherever she goes. Instead she must learn to stand and fight.

But when the half-human, half-Shadow Warrior Lucas Gray is sent to spy on Evie and then ordered to kill her before she can fulfil a dangerous prophecy, their fates become inextricably linked. The war that has raged for one thousand years between humans and demons is about to reach a devastating and inevitable conclusion. Either one or both of them will die before this war ends.

If your life becomes bound to another’s, what will it take to sever it?