The Eminence in Shadow, Vol. 6 (light novel)

A mysterious cabal called the Thirteen Nightblades has been running things in the Midgar Kingdom from the shadows, and its members have concealed all manner of crimes through their connections to the Cult of Diablos. How impressi-no, how terrible. These dastardly nobles commit so many evil deeds, and yet justice always evades them. That's when a serial killer known only as "Jack the Ripper" appears, plunging the corrupt capital into chaos! In order to eradicate evil, an even greater evil is needed...