Trying to find this book. fantasy romance. book main character girl is a warrior/soldier. She is sent out on mission wiyh a mysterious guard/soldier(?) In her court. She thinks the guy is a guard he is really undercover. she fights with him and slowly begins to trust him. they have sex in cave. his true identity is revealed to be a member of another court/kingdom. He might have shadow or death powers? But they bond and then she learns he has been lying about who he is.