Evil exists. Evil walks the streets. And evil has spawned a diabolical new disciple in this white-knuckle thriller from New York times bestselling author Tess Gerritsen. Peccavi. The Latin word is scrawled in blood at the scene of a young woman's brutal murder: I have sinned ... It's a chilling Christmas greeting for Boston medical examiner Maura Isles and Detective Jane Rizzoli, who swiftly link the victim to controversial celebrity psychiatrist Joyce O'Donnell--Jane's professional nemesis and member of a sinister cabal called the Mephisto Club. On top of Beacon Hill, the club's acolytes devote themselves to the analysis of evil: Can it be explained by science? Does it have a physical presence? Do demons walk the earth? Drawing on a wealth of dark historical data and mysterious religious symbolism, the Mephisto scholars aim to prove a startling theory: that Satan himself exists among us. With the grisly appearance of a corpse on their doorstep, it's clear that someone--or something--is indeed prowling the city.