The Joker wandered through New City.
He had a goal that none of Arkham's lunatics knew about. He wanted to investigate something that had long puzzled him.
The Joker felt that he had lost something.
This Joker could be called the darkest and most insane Joker, because he had no origin story.
When the Joker of the New 52 universe first appeared, he was Red Hood One. This Red Hood was not Jason, but the leader of the Red Hood Gang, the head of a criminal organization.
The Red Hood Gang targeted Bruce Wayne and drove him into danger several times, until Batman appeared.
Batman defeated Red Hood One. He fell into a chemical vat and became the Joker. That was the entirety of the New 52 Joker's backstory.
Without the worst day ever, the New 52 Joker's childhood was filled with misfortune. But it could also be said that he was simply born bad. He shaped his own evil and chose the path he would walk.
After the Joker appeared, he became Batman's number one mortal enemy, creating countless major cases throughout Gotham. Everything had been normal, until one day it was not.
Recently, the Joker had been having a dream. He dreamed that he shattered into tens of thousands of pieces, floating among the stars before reassembling.
He dreamed that his current self was not himself, that his former self had not been himself either, and that he had an even more distant past. But now, all of it had vanished.
Without a trace, too subtle to notice. No one around him had realized it, but the Joker's genius and sensitivity, equal in measure to his madness, made him realize that he must have lost something.
What came next was simple. He needed to look for the differences