Captive Allure by Imogen Sera

Petra has only ever been worried about her sister, and being kidnapped by a smoke monster and separated from her for the better part of a month certainly hasn’t helped matters. After her harrowing escape, she ends up sick and exhausted and stumbling into the most handsome man-well, if it weren’t for the horns and fangs and creepy eyes-that she’s ever seen.
In retrospect, locking his mate underground was probably a bad idea, but Ronan wasn’t really himself at the time. He knows better now, he really does, but there’s just no good way to tell her that he was the one who imprisoned her.