He made a deal with Ra's al Ghul during his incarceration, trading the formula for his Joker Venom in exchange for a quantity of mutagen. Shredder and Ra's al Ghul broke him out of his cell after giving him the mutagen, and the Joker proceeded to free all the Arkham inmates, just before mutating them. He himself turned into a massive mutant cobra.
When Batman and the Turtles arrived to save hostages from him, the Joker was dismayed when his new appearance did not spark any interest from Batman. When Mr. Freeze froze Batman to the floor, the Joker decided to inject the trapped superhero with a mixture of Joker Venom and mutagen, which would both mutate him and drive him insane. When a newly-mutated Batman went berserk, the Joker was delighted.
He then fought Batgirl for a time, and was defeated by her, losing all of his teeth in the process. He was left for the police to apprehend and de-mutate back to his human form.
- The Joker's voice actor, Troy Baker, also voiced the character in Batman: Arkham Origins and Batman: Assault on Arkham.
Joker - Turns out, when you combine the ooze with my little tincture, you get something new. Something that not only transforms you... but drives you mad.