Scattered all over Arkham City are newspaper clippings with the headline "Criminal Couple Gunned Down" and a smaller heading on the left that says "Boy Watches as Parents Die!". These are clear references to Prometheus, one of the famous villains in the DC universe, and a dark reflection of Batman.
Also, inside the old Gotham City Police Department building it's possible to find an Arkham Medical Examination Record for Prometheus, which states he "passed" and has been released on November 12th, 1982.
Contributed by SRJ