The Mephisto Club by Tess Gerritsen
Can you really see evil when you look into someone's eyes? In a rundown house, a woman has been dismembered in an act of carnage that leaves veteran cops in shock. Drawn on the wall, in blood, are ancient symbols, and a mirror-image word in Latin that, translated, says: "I have sinned." Then a second woman is found butchered on Beacon Hill, just outside the home of the leader of the Mephisto Club, a secret society dedicated to the study of evil. On the door yet more ancient symbols have been scrawled. This is evil that the Boston PD has never encountered before. And the only way Maura Isles can defeat it is by turning to the people who understand the devil himself.