Sin Undone (Demonica, Book 5)

As the only female Seminus demon ever born, master assassin Sinead Donnelly is used to being treated like an outcast. She spent decades enslaved, and now vows she'll die before she'll relinquish her freedom again. Then Sin's innate ability to kill her enemies goes awry: She creates a lethal new werewolf virus that sparks a firestorm of panic and violence. Half-werewolf, half-vampire Conall Dearghul is charged with bringing in Sin to face punishment for the plague. And she's no stranger: He's bound to her by blood, and the one sexual encounter they shared has left him hungering for her raw sensuality. Worse, Sin is the underworld's most wanted and Con soon learns he's the only one who can help her . . . and that saving her life might mean sacrificing his own.