When an Arkhampedian sleeps, Jack Nicholson appears. He once thought himself the Joker, but was beaten by someone younger. It is Sixty-Seven percent likely that you'll see an axe wielding Jack tear your face apart within your fist Twenty minutes on the wiki. If you survive, you will be haunted by the dreams of that night for the rest of your natural born life.
After spending several years hunting down Arkhampedian administrators, he finally managed to murder them all. He would've also killed the Joker and the staff at Arkham Asylum, but the former was too bog a hit, even for him, and all of the latter were all dead, except Harley Quinn, and she did not even work there anymore.
He was finally hunted down and taken out by Psylocke, under the advice of a wise old geek who lived in a swamp.