Satan's Sword (Imp, #2‪)‬

Samantha Martin is an imp, bound by an angel who allows her to live among the humans . . . as long as she follows his rules. It’s never easy for an imp to follow the rules, especially now that Sam’s brother, Dar, has found himself in hot water. He needs her help to retrieve an artifact from the vampires, or the powerful demon he owes a favor to will enslave and torture him for centuries. It should be a simple courier job, but with demons nothing is simple. Sam reluctantly attempts to help her brother, trying not antagonize the vampires or the demon gunning for him, all while chafing to comply with the restrictions her angel has placed on her as a bound demon.