Warning: contains spoilers for Superman: Son of Kal-El #13!
In a vision shown to Superman, the Joker is proved to be the greatest threat to the Justice League, as his signature weapon wipes out the team with very little effort. The Joker tends to keep his schemes Gotham-centric, but with one small twist to his famous Joker Toxin, he could easily wreak havoc on the whole world. Not even Superman could stop the Joker if he chose to care about destroying anything beyond Batman.
The Joker has plagued Batman since 1940, when he made his first appearance in Batman #1 by Bill Finger, Bob Kane, and Jerry Robinson. Though the super-villain occasionally teams up with other villains outside of Gotham for the resources he requires, he primarily sticks to Gotham and the larger Bat-Family. His main - and most famous - weapon is his Joker Toxin, which causes the victim to break out in riotous laughter before quickly dying. Meanwhile, over in modern-day Metropolis, Jon Kent's Superman is currently battling the Gamorran president Henry Bendix, who continues to threaten Jon and his loved ones with his army of mind-controlled post-humans.
The Joker Gas is used by Bendix - in a vision of the future - with deadly consequences in Superman: Son of Kal-El #13 by Tom Taylor, Nicole Maines, Clayton Henry, Matt Herms, Marcelo Maiolo, and Dave Sharpe. The just-introduced Dreamer presents Superman with a vision of an impending future in which Bendix drops a post-human made of Kryptonite-infused Joker Toxin on Gotham. When reinforcements are called in, not only do countless civilians die, but so does much of the Justice League and other heroes - including Batman, who Bendix shoots in the head. The Kryptonite in the gas debilitates Superman and leaves Bendix free to take control.
Even though Dreamer presents this future vision to Superman in order to stop it from coming to pass, these events prove that the Joker's gas - with a few adjustments - could bring even the Justice League to its knees. Major universe-wide threats are often cosmic in scale (like the Great Darkness in the current Dark Crisis event), but here it's made clear that with a little ingenuity, Joker could equal the biggest threats the Justice League has faced. This vision also proves one thing to be true about the Joker: he only cares about Batman. A villain like the Joker could have created such a plan many times over, but he doesn't care about world domination so much as causing chaos for Batman and Gotham.
The Joker could be the entire DC Universe's most threatening villain, but his single-minded devotion to causing chaos for Batman holds him back to the extent that he only messes with other heroes to cause the Dark Knight trouble. But if other, more world-domination focused villains - like Bendix, in this possible future - got their hands on the Joker Toxin, they could destroy the world as the heroes know it in mere seconds. Fans now know how Joker could kill off Superman and the entire Justice League, if only he could get over his blinkered obsession with Batman.
Check out Superman: Son of Kal-El #13 available now from DC Comics!