Bound by Vengeance. A Sentinel Wars Novella (A Penguin Special from New American Library)

The guilt that Defender Dakota Kacey feels about her brother's death is all-consuming, making her determined to find and kill the demon who murdered him. But the leader of the Defenders is worried about what Dakota is capable of, and calls in Liam Lann to keep an eye on her. Dakota knows what kind of power a Theronai like Liam possesses, and agrees to bind herself to him if he'll help her exact vengeance.
But a bond to a Theronai is not a decision to make lightly, and as the search for the killer intensifies, Dakota will have to learn to trust Liam completely, or risk losing everything.