Warning: SPOILERS for Justice League #35
DC Comics has a long history of "Elseworlds" stories imagining what their characters would be like in other universes, and of the coolest of them all imagined a version of Gotham by Gaslight, introducing a Victorian-era Batman fighting Jack the Ripper; but now he, and his whole universe, have been erased from existence. In Justice League #35 the team's new archenemy, Perpetua, wiped it out, ending, in her words, "Trillions of lives in a single instant."
So who is Perpetua, and how can she be so powerful? Introduced as part of the epic Year of the Villain storyline Snyder and Tynion are currently writing, she was one of the Super Celestials, created by the Source of all existence to create new types of life. Perpetua had her own plans, though, and wanted to create a form of eternal life: "apex predators" she could use as a weapon against her masters. Her creation became the 52 worlds of the DC Multiverse, and the children created to enforce her will: DC icons like Mar Novu the Overmonitor, and Mobius the Anti-Monitor.
Unfortunately, in the events of Snyder and Greg Capullo's Dark Nights: Metal, the Source Wall containing Perpetua cracked, allowing the Totality containing all her power to appear on earth. With some help from Lex Luthor and his Legion of Doom, Perpetua broke free. Now she's ready to remake the multiverse in her own image, and she starts by destroying the Gaslight universe for the crime of choosing justice over her own, more nihilistic ideal of doom.
The world of Gotham by Gaslight will be missed. The original story, written by Brian Augustyn, with art by Hellboy creator Mike Mignola, and P. Craig Russell, was the first of DC's Elseworlds series of graphic novels, popularizing the concept before the name had even been coined. As well as moving Batman a hundred-odd years into the past, it also gave him the opportunity to meet with real historical figures: he's analyzed by Sigmund Freud and chases Jack the Ripper when he tries to set up shop in Gotham, who in the universe had been a Civil War field surgeon alongside Bruce Wayne's father, Thomas.
Augustyn wrote a sequel, Master of the Future, with art by Eduardo Barretto, where Batman fought the titular air pirate and met Buffalo Bill Cody. Various DC characters made it over to this universe, most recently in the Convergence: Shazam! miniseries by Jeff Parker, Evan "Doc" Shaner, and Jordie Bellaire. In this series, Superman's alien enemy captures cities from across the multiverse and forces them to battle each other for their own survival. Shazam's Fawcett City faces off with the Victorian-era Gotham, giving Shaner a chance to show off this universe's version of the classic Batman rogues' gallery.
Gotham by Gaslight even received an animated adaptation, with a cast including Doctor Sleep's Bruce Greenwood as Batman and Dexter's Jennifer Carpenter as Catwoman who, along with many other classic Batman characters, were added to the cast of the original comic. Now that this universe has been destroyed, there's no telling where Perpetua could go next. Any world might be under threat -- even DC's Prime Universe itself.
Justice League #35 is available now at your local comics shop.