In 1995, this crossover from Marvel Comics among the X-Titles began when Legion, son of Professor Xavier, travelled back in time to kill Magneto, but ended up killing his own father instead. This story, itself a minor crossover called Legion Quest, led to the main Age of Apocalypse crossover.For four months the X-titles were replaced by miniseries that took place in the Alternate Universe that had come about for want of an Xavier. This Bad FutureVillain World was a dystopianCrapsack where heroes were villains, villains were heroes, and Anyone Can Die. The non-mutant comics weren't part of the crossover, though a two issue miniseries X-Universe did show what happened to the non-mutant Marvel heroes in the new history. Scott Lobdell directed the overall story arc and some of Chris Bachalo's best work is featured in this series. The series were renamed as follows:
Uncanny X-Men - Astonishing X-Men
X-Men - Amazing X-Men
X-Men Unlimited - X-Men Chronicles
Wolverine - Weapon X
X-Force - Gambit & The X-Ternals
X-Factor - Factor X
Excalibur - X-Calibre
Generation X - Generation Next
Cable - X-Man
The Constant was Bishop. Hailing from an alternate future, he was already a chronal anomaly and thus unaffected by the change. He was the only one who remembered the world 'as it should be' and convinced the heroes to Set Right What Once Went Wrong, even though it would mean that their own timeline would cease to exist. It turned out that the world was doomed anyway, as both hemispheres would have been leveled by nuclear attacks. Some characters managed to escape the closing of the temporal singularity and entered normal Continuity. One who failed to make it back was the remarkably popular Blink, the second of her lamented deaths. Death may be cheap, but she did not return until five years later in Exiles, a series about a team of characters who are all Unstuck in Time.On the tenth anniversary a sequel miniseries was produced, which isn't very popular among fans; among other things, it shows little evidence of the devastation evident in the original series. There was also a "What If" story pitting the survivors of the original series against Galactus. An arc in Uncanny X-Force paved the way for a 2012 series for the survivors.A shining example of the Darker and Edgier comics of The Nineties: a literal post-Apocalyptic warzone.
Apocalypse Not: The ten-year revival series makes the world look much less nasty in comparison to the original run.
A Real Man Is a Killer: Colossus expects the younger mutants to kill one another during training to ensure survival of the fittest.
Badass Normal: Gwen Stacy stands out the most, compared to her counterpart in the normal continuity, but many other other non-mutant Marvel heroes (who managed to escape Apocalypse's human cullings...Peter Parker and half of the Fantastic Four did not) qualify as well.
If Holocaust had thought about where his name came from for just a minute, it would be obvious why he's on Magneto's hit list.
Black and Gray Morality: Forget about Magneto. In a setting where the mutant Nazis have already conquered or wiped out half the world and the remaining humans are willing to nuke them in self-defense, Sabretooth and the Sentinels are good guys.
A Lighter Shade of Grey: That said, some of the "former" villains are vastly more sympathetic in this setting - Magneto is utterly haunted by Charles' death and thus tries his absolute damnedest to live up to the dream in a world that is far more actively hostile to the concept, and AoA Sabertooth was famously more jovial, likeable and friendly than his 616 counterpart had been up to that point. Both of these turns were so liked (in addition to the Mags-Rogue relationship) that elements of the AoA characters were adapted into the primary versions of the characters after AoA came to a close.
Blessed with Suck: Averted, in normal continuity, Chamber's power causes most of his chest and face to melt off, but in this reality, he simply had a hole drilled in his chest to let the energy escape.
Breakout Character: Blink, easily the most popular character to come out of the event, who started as an Ensemble Dark Horse. This eventually resulted in her leading a team in Exiles due to fan demand to bring her back, and, again, she is easily the most popular character in that series.
Canon Foreigner: Among others, Charles Lehnsherr, Dead-Eye, and Switchback. The only thing the AoA Arclight, Bastion, and Wolverine share with their classic Marvel Universe namesakes are the names. The classic Arclight is a woman (Ao A Arclight's a man), the normal MU Bastion appeared as a mutant-hater afterAo A, and the AoA version of the classic Wolverine goes by the name Weapon X. Sugar Man had no known counterpart in the classic Marvel Universe, but as noted below he, plus Dark Beast, X-Man, and Holocaust (whose respective counterparts are Beast, Cable, and the recently-revealed Genocide), did literally immigrate from the AoA reality to the classic universe in the finale.
A version of Abyss—one of Apocalypse's horsemen—later appeared in the normal Marvel Universe, but due to Apocalypse not ruling the world, he was a hero and Damask's 616 counterpart is Excalibur villain Emma Steed. Oh, and thanks to recent events in Uncanny X-Force, there's now a 616 version of Holocaust, calling himself Genocide.
Holocaust, Sugar Man, Dark Beast, X-Man, AoA Sabretooth and AoA Blink are literal examples of this; the former four found their way into the classic MU during the finale (X-Man interacted with his counterpart, Cable, a few times and Dark Beast once impersonated his counterpart, the classic Beast in the lead-up to Onslaught) and the latter two, via Exiles. Given Holocaust died, he's unlikely to meet his newly-revealed 616 counterpart, Genocide, unless he's resurrected.
More recently, AoA Blob, who joined Daken's Brotherhood of Mutants and AoA Nightcrawler, who has joined X-Force
Catch Phrase: Apocalypse's constant referrals to the "chosen" and the "forgotten".
Co-Dragons: The Horsemen of Apocalypse (Sinister, Holocaust, Abyss and Prelate Rasputin). Former Horsemen (Candra, Gideon, War, Death, Maximus/Death II, Bastion, Sabretooth).
The Elite Mutant Force serve as Co-Dragons for Sinister (Cyclops, Havok, Cannonball, Amazon, Northstar, Aurora, the Bedlam Brothers, Marrow, the M Twins)
The Marauders, human terrorists serving Apocalypse: Dirigible (Wilson Fisk), Red (Norman Osborn), Arcade and Owl.
The Reavers, cyborgs enhanced with Apocalypse's technoorganic virus.
Death World: Several continents have been wiped out by nuclear weapons, most of the rest of the planet is an irradiated wasteland, genocidal death squads hunt down and wage war on the surviving human race and "race-traitors", dissenters and opponents of the tyrannical regime can expect to be thrown into concentration camps to try and survive sadistic experiments, and North America is divided between the Evil Overlord and his most trusted and dangerous lieutenants, who rule their territories as personal kingdoms. The world is intentionally crapsack as Apocalypse believes everyone has to earn the right to live and will come out stronger for it by surviving and prospering on this nightmare planet.
Did They or Didn't They?: Fans were convinced that Rogue and Magneto hooked up in a previous storyline (despite Word of God), perhaps the reason why they were a genuine couple in this series.
Don't You Dare Pity Me!: Quicksilver learned that his father had been kidnapped by his worst enemy, his half brother had vanished, and a virtual stranger had also been captured. He had to decide to rescue the stranger. When his girlfriend Storm tried to sympathize, he refused to talk with her because if he thought of what he was doing, he would not be able to do it.
Everybody's Dead, Dave: This is the nature of the report Colossus gives about Generation Next to Blink and the rest of the X-Men—his entire class of students (for those keeping score, that's Chamber, Vicente, Mondo, Husk, and Skin) was sacrificed to save Ilyana and repair the timeline.
Evil Overlord: Apocalypse himself is one of these in this story. Holocaust and Mikhail have similar plans of their own.
Numerous characters are on different sides in this reality, including Sabretooth, Exodus, Cyclops, Havok, and Dark Beast, who even crossed over to the main Marvel Universe and became a recurring X-Villain.
In universe examples include Cyclops and Strong Guy among others.
Wolverine as of the Dark Angel Saga in Uncanny having become the heir to Apocalypse, with a Brainwashed and Crazy Storm, and Bobby Drake.
Future Badass: Almost everyone in the series is a future badass version of themselves; the one glaring exception is Illyana Rasputin, who never went to hell in this continuity and is therefore a little girl rather than a teenaged demon-sorceress.
Go Through Me: During a massive battle, Colossus sees that his little sister is in trouble and rushes to save her, with gusto. Iceman steps up and says something along the lines of "No can do pal, if you want to get out of here you'll have to go through m-"... and the next panel shows Colossus smashing Iceman into a million pieces. Though that wouldn't actually kill him now after some power Ret Cons.
Irony: Magneto has always been concerned about a human genocide against mutants. Instead, there ends up being a mutant genocide against humans.
The Holocaust almost killed Magneto. Magneto almost-kills Holocaust
Kill 'em All: As one would expect from a dystopian alternate timeline, numerous Marvel characters were killed off as part of the backstory, and many more die over the course of the main story. And then, in the finale, the human resistance sends a massive airship fleet to Apocalypse's domain to Nuke 'em All. Even though Apocalypse has already been killed. Later, a What If? story shows the handful of survivors, human and mutant, trying to rebuild...and then Galactus shows up.
Marked Bullet: Mystique, also the source of the title 'X-Calibre'.
Sinister (without the "Mister") betrays Apocalypse and seeks to destroy him because Sinister believes no lifeform will survive his plans.
Holocaust is also implied to have more extreme plans.
Super Power Meltdown: X-Man was engineered by Sinister to eventually (and literally) burn out from the strength of his psionic power.
During his adventures bopping around the multiverse, he finally managed to get this fixed.
The Time Traveller's Dilemma: Fixing things to how they should be means the end of the line for everyone in this dimension, which gives Magneto pause when he considers the son he has with Rogue.
Truce Zone: Heaven, the nightclub run by Warren Worthington III, he himself attempts to play the True Neutral role, providing entertainment to the elite of Apocalypse's regime, but also information to "terrorists" like the X-Men or Gambit's X-Ternals.
Unexplained Recovery: Even though the usual "This is a 'What If' so let's kill everyone we can' rules apply, one character actually enjoyed a normal comic book resurrection, albeit as an after the fact Retcon brought about by editorial bungling. Cyclops was intended to die in the end, but as many fans pointed out, the depiction of his death makes no sense; Havok's power shouldn't be able to even hurt him, much less kill him. So in the epilogue miniseries, he was established to be alive again (though he never appears).
Not counting the ten-year revival (many fans don't), Psylocke is the most notable no-show, though Wild Mass Guessing suggests that she was lobotomized as part of Apocalypse's Brain Trust (a collection of disembodied telepathic brains that serve as Big Blue's primary psychic defense), thus explaining her brother Brian's almost manic dedication to bringing Apocalypse down.
The Psylocke in the ten-year revival is most likely Kwannon, the Chinese telepathic assassin who body-switched with Betsy Braddock in the main universe, who remained in possession of her original body in this reality.
The lack of magic or magic users like Doctor Strange or X-Men ally Amanda Sefton has also been noted. Strange possibly died during the culling, when he still was merely a human surgeon, along with Peter Parker, Reed Richards and Johnny Storm. And Cap's probably floating in a block of ice, somewhere in the Artic.
World of Badass / Took a Level in Badass: The Ao A-verse is truly an awful place, dominated by the Darwinist decree of 'Survival of the Fittest.' And yet, this same philosophy creates a 'verse where nearly everyone is even more badass then they were in the original timeline, in one way or another. Thought that Sabretooth was already a tough mo-fo? Wait until you see his first fight with Holocaust. You don't think Iceman is living up to his full potential power-wise? Let Ao A Iceman show 616 Iceman how it's done. And Wolverine manages to kick just as much ass, if not more, with just the one claw. The list goes on and on, which makes sense, considering just what it takes to survive in this hellhole.
Wolverine Claws: In addition to Logan, Shadowcat wears a gauntlet with three retractable claws.