WARNING: This article contains SPOILERS up to Dark Nights: Metal #6
Batman has crossed an unthinkable line with The Joker in Dark Nights: Metal #6. The final issue of the best-selling and critically acclaimed mini-series sends Batman up against the ultimate nightmare 'What If?' in the parallel universe version of Bruce Wayne who became Joker - also known as The Batman Who Laughs.
The Batman Who Laughs is the leader of the The Dark Knights - a collection of alternate-universe nightmare Batmen from worlds where the forces of evil were triumphant - who have been the primary antagonists of Dark Nights: Metal. Hailing from Earth -22, where The Joker finally forced Batman to kill his grinning nemesis, Joker's corpse released a nanotech toxin that rewrote Batman's brain chemistry to make him think just like the Clown Prince of Crime. The end result was a clinically insane villain who paired Joker's unpredictable violence with Batman's training and gift for tactics.
The Batman Who Laughed murdered his entire family of heroes, but in Metal #6 he learns the only thing more dangerous than a mix of Batman and Joker... is when they join forces.
Faced with an enemy who is capable of anticipating anything that he could think of - because of the simple fact that he is also Batman - the Batman of Earth Prime (the one getting ready to marry Catwoman) is forced to take drastic action. Logically, Batman reasons, the only way he can win is by doing something that he could, quite honestly, never imagine himself doing.
This leads to the unthinkable: Batman partners with The Joker to take on The Batman Who Laughs as a twisted Dynamic Duo! A new, terrifying, nightmare team-up of their own.
At first The Batman Who Laughs is confused, thinking that The Joker showed up on his own, finding the prospect of killing two versions of Batman irresistible. The truth is much more twisted. As The Joker explains, he's driven only by the pure irony and utter insanity of working with Batman to save all of reality. Why? Because nobody would ever expect The Joker to do it. And when reality itself is being torn to pieces, you can always count on The Joker to do what's least expected.
As entertaining a twist as Metal #6 offers, Batman and The Joker joining forces isn't entirely unprecedented. The two briefly joined forces in the classic story The Demon Laughs, after Ra's al Ghul attempted to form an alliance with The Joker. When Ra's tired of The Joker's antics and killed him, Batman resurrected his greatest enemy using a Lazarus Pit and was astonished when Joker emerged fully sane, the madness usually instilled by The Lazarus Pit having instead temporarily restored his faculties. The Joker also famously rose to the occasion and fought The Red Skull in the Captain American & Batman Elseworlds, after realizing that his partner-in-crime really wasn't playing at being a Nazi.
A shocking end to a shocking comic event. All things considered, this historic team-up may be forgotten when compared to all the ways Metal changes DC's Comic Universe from here on out.