The biggest comic book events all have a habit of completely rewriting their respective universes' continuity. Although, with that said, it's hard to say what the most impactful comic book events are, as most of them have a follow-up event in a few years that is tasked with the sole purpose of rewriting the event itself. Especially in the DC part of the industry, it seems like every event after Crisis on Infinite Earths has been an attempt to rewrite Crisis on Infinite Earths in one way or another.
Marvel, for its part, doesn't like to retcon anything officially, but that doesn't mean someone in a story won't use their powers to magically alter something in a character's life. Whether that's God Emperor Doom creating a new universe, or Mephisto robbing Spider-Man of his one true love, they're not fooling anyone over at Marvel. The reality-shaking events are still happening; they're just a little trickier about it.
What Was The Threat: The in-universe threat was that the Anti-Monitor was on a mission to destroy as many Earths across the multiverse as he could. The real-world threat facing publishing was that the DC line now had literal decades of continuity that made it incredibly unfriendly to new readers.
How Did The Heroes Save The Day: The Monitor (not to be confused with the Anti-Monitor) recruited heroes from across the multiverse to band together and stop the Anti-Monitor. He planned to merge the multiverse back into one universe, thus making it strong enough to defeat the Anti-Monitor (and contained enough to be enjoyed by new readers).
What Changed Afterwards: Instead of a multiverse, the DC universe moving forward was now one universe, with one version of each character. Also notable, Barry Allen and Supergirl perished and were absent from comics for years to come. The stated goal of Crisis on Infinite Earths, as explained by its writer Marv Wolfman, was that "by series' end, DC will have a consistent and more easily understandable universe to play with."
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What Was The Threat: In Jonathan Hickman's acclaimed run on Avengers and New Avengers, the heroes (and some villains) on Earth began to realize that there were "incursions" happening across the universe in which different realities were smashing into each other and being eliminated. To prevent the destruction of their reality, Doctor Doom was able to pilfer the power of the Beyonders to turn himself into a god. Then, as reality blinked out of existence, he formed a new world, called Battleworld, which contained segments of different realities that he ruled over completely.
How Did The Heroes Save The Day: While the inhabitants of Battleworld didn't remember their previous history in their own realities, there was a group of stowaway heroes from the 616 universe that did. These heroes mounted a resistance that eventually ended with Black Panther wielding an Infinity Gauntlet against Doom and Reed Richards convincing Molecule Man that he would be better off in charge instead of Doom.
What Changed Afterwards: Moving forward, the biggest change was that various characters from alternate universes - most importantly, Miles Morales and his supporting cast - now lived on the new Marvel Earth-Prime. Following the storyline's end, the Fantastic Four title went on a brief hiatus as the characters were out in the cosmos using the powers of Molecule Man and Franklin Richards to rebuild the universe.
Hal Jordan Went Bonkers And Attempted To Rebuild The Universe In 'Zero Hour'
What Was The Threat: Hal Jordan, after being driven mad and becoming a villain known as Parallax, attempted to reshape the timeline to remove the destruction of Coast City. Once again, there was also a publishing threat the event aimed to deal with, which were the leftover continuity issues caused by Crisis on Infinite Earths.
How Did The Heroes Save The Day: The other heroes eventually managed to band together to stop Parallax. He was weakened by the Spectre, allowing Green Arrow to destroy him. But, the heroes didn't stop him in time to totally stop all of the changes to the timeline. Their fighting actually ended up triggering a new big bang that rewrote their histories.
What Changed Afterwards: CBR described this event as one of the most collaborative crossovers ever, as the main point of the event was to let the writers on various titles tell the event writer, Dan Jurgens, what they wanted to be changed in their respective character's histories. All of the various Hawkmen became one singular Hawkman, some of the JSA members (whose ages had become more than a little confusing) were wiped out, and the Legion of Superheroes was wiped out, as well.
Wonder Woman Harnessed The Powers Of A God To Make All Continuity Matter Again In 'Death Metal'
What Was The Threat: The event began with a god-like being named Perpetua reshaping reality and gifting the Batman Who Laughs the main-continuity DC Earth to rule over in his own terrifying image.
How Did The Heroes Save The Day: The Batman Who Laughs achieved god-like powers and started his own conflict with Perpetua. He eventually won, and then sent his armies to Earth to bombard the combined might of the DC universe's heroes and villains. Meanwhile, Wonder Woman dipped her Lasso of Truth into the World Forge, which allowed everyone in the universe to begin to remember their full histories. Diana then engaged the Batman Who Laughs in a clash of gods that ended with other members of Perpetua's race restoring the DC multiverse.
What Changed Afterwards: In some ways, Death Metal was something of an anti-event. While previous events, like Crisis and Flashpoint, attempted to streamline continuity by making different realities and storylines cease to exist, Death Metal did the exact opposite. It ended with the creation of an omniverse, where all histories and stories matter and exist. The extent of this event, which just ended at the start of 2021, hasn't fully been seen yet in the universe, but the possibilities seem endless at this point. The reality and continuity that has existed since 2011's New 52 launch has now ended, and those stories now exist along with every other story DC has ever told.