The Joker has done a lot of heinous shit to other people, but rarely has he messed with the man in the mirror. Historically, it's been much easier to get his jollies off by torturing someone else, whether it be Batman, Harley Quinn or any henchman dumb enough not to know that working for the Joker is essentially a terminal illness.
Someone at DC might have noticed this aspect of the character, because right when the rebooted New 52 universe kicked off, the Joker cut his own face off and pinned it to a wall.
After that, Joker wasn't seen for a while. When he emerged from his self-imposed leave of absence, Joker was wearing his own decomposing face as a mask, with the sinewy facial tissue clearly visible underneath his fleshy garment.
Did the Joker have a dastardly new plan to go along with his ill-advised cosmetic surgery? Well yeah, of course, but the particulars are ultimately kind of meaningless when you can't stop staring at the flies buzzing about his rotting fleshmask. The nightmarish new look begged the question: "Why would anyone ever do that to themselves?" That's the exact question Joker wants you to ask. If he's willing to mutilate himself just for fun, imagine the horrors he could inflict on someone he actively doesn't like.
Though most prominently featured in Scott Snyder and Greg Capullo's epic run on Batman, the most unsettling Joker moment came from Tomasi and Gleason's Batman and Robin.
Yep, that's the Joker "turning his frown upside-down" by flipping his maggot-infested faceskin around 180 degrees. Best to cut off your eyelids now, because you're not going to see anything else when you go to sleep.
If there's anything comics love more than time-travel, it's alternate universes caused by time-travel. Such was the case for Flashpoint, a huge DC event that eventually gave way to the New 52. This alternate universe imagines a world where the Flash's mother never died, and the Butterfly Effect has caused massive ripples throughout the timestream. In the Flashpoint reality, Aquaman is at war with Wonder Woman, Superman's baby pod landed in Metropolis and not Smallville, and everyone continues to not care about Cyborg.
One of the most notable changes came during that fateful night in Crime Alley. Thomas and Martha Wayne weren't killed in a mugging gone wrong, but instead there was another casualty: little Bruce Wayne. This left a vengeful Thomas Wayne to become an older, grizzled Batman. But Thomas took it pretty well compared to Martha.
Distraught at the loss of her son, Martha Wayne goes insane, slashes her face and becomes the Joker. Even though the Thomas Wayne version of Batman is much more likely to murder his enemies with guns, he still has a hard time coming to grips with the idea of killing his former wife -- no matter how insane she may be. This new Joker knows this, and uses her power to get away with some pretty sick stuff. She tricked Comissioner Gordon into killing Harvey Dent's daughter, and then slit Gordon's throat.
Much has been said about how we should never know Joker's true origin, that taking away the mystery makes the character less frightening. But with Martha, it works -- we know that she's lashing out at the world for taking her son, and tormenting her husband for his failure.
During the course of the Flashpoint miniseries (which you can also catch as a DTV animated movie), Batman learns that he can restore the past and save Bruce's life. It sounds like a great idea to Martha, until she learns the details.
It's touching to see a moment of sanity between Batman and Joker, even in this warped context. But it goes from heart-wrenching to downright creepy when they embrace.
To be clear, Batman and Joker are always about six inches from a hardcore makeout, but it's never happened until now. On one hand, it's nice to see the archenemies finally consummate their relationship. On the other hand, you know his tongue probably went through her faceholes at some point.