Christopher Nolan wasn’t lying when he said he wouldn’t include Heath Ledger’s Joker in THE DARK KNIGHT RISES. There’s no posthumous appearance via stock footage or CGI (both of which were rumored), no voice cameo (they could’ve easily snuck Ledger’s voice into one of the many flashbacks), no brief glimpse during when the prisoners escape Blackgate Prison (maybe seen conspiring with Tim Booth’s serial killer Zsasz who was also MIA), no nothing. Hell, he’s not even mentioned in passing or even hinted at. Harvey Dent is name-checked over a dozen times. The Joker? Not once.
But not to worry, Bat-fans! The film’s novelization contains a brief mention of The Joker and his whereabouts following the events of THE DARK KNIGHT. No one seems to be totally sure if what’s below was actually ever in Nolan’s script (doubtful) or if it was made up on the fly by author Greg Cox (likely) but it's pretty cool that fans have an answer as to what happened to the Clown Prince Of Crime.
Here’s the excerpt: Now that the Dent Act had made it all but impossible for the city’s criminals to cop an insanity plea, it (Blackgate Prison) had replaced Arkham Asylum as a preferred location for imprisoning both convicted and suspected felons. The worst of the worst were sent here, except for the Joker, who, rumor had it, was locked away as Arkham’s sole remaining inmate. Or perhaps he had escaped. Nobody was really sure. Not even Selina.
And here's a picture of the excerpt to prove it's real: