Monster Without Mercy by Cynthia Eden

Anyone can be tempted…
Xavier Hollow is the king of destruction. A shifter, a vampire, a demon—every nightmare you have—all rolled into one very smoking hot package. Xavier snaps his fingers, and the world falls at his feet. His enemies tremble in fear. And the women who want him? They tremble with yearning. Life (and death) is one fabulous ride…until a certain very old curse kicks in for him. A curse that threatens to take away all of his dark power.

He needs someone good…to fall for him. STAT.
Three days. That’s how long Xavier has to tempt one Mercy Josephine. Innocent, pure, the actual descendant of an angel, she’s all the things that normally give him hives—literally. He’s allergic to good and just can’t stand it. Committing a “good” act causes Xavier physical pain. The bigger the act, the more intense his suffering. But, for Mercy, he’ll have to pretend to be someone very, very different. Someone who is—unfortunately and ever-so-disgustingly—good.