The X-Men's "Age of Apocalypse" is still the best What If? story in Marvel Comics history. Marvel has always recognized the infinite potential offered by "What If?" stories. These act as a window into the world as it might have been, allowing writers to offer fresh and innovative takes on classic characters. Many of these stories have established a template for plots that made their way into the mainstream comics, or even the movies, most notably with Jane Foster becoming the Mighty Thor.
In 1995, Marvel launched what proved to be their most ambitious What If? story of all time. It spun out of a story called "Legion Quest," in which Charles Xavier's son Legion went to insane lengths to prove his love to his father. He correctly realized the mutant race had divided over the competing visions of Magneto and Xavier, and attempted to use the power of time travel to rewrite history by killing Magneto long before the schism ever began. Unfortunately it went wrong, and he accidentally killed his own father; the demonstration of Legion's power prompted the monstrous Apocalypse to reveal himself to the world decades before any superhero teams had been formed to oppose him.
"Age of Apocalypse" was one of the boldest stories in the history of Marvel Comics. It was published at a time when the X-Men were Marvel's best-sellers, but in spite of that Marvel essentially shut down the entire line for four months and replaced it with an alt-universe story. In this timeline, Magneto had founded the X-Men in honor of his late friend Charles Xavier, and he had been fighting a losing battle against Apocalypse for years. Fortunately, one survivor from the previous reality - an already time-displaced - Bishop - retained his knowledge of true history. When Bishop encountered the X-Men at last, Magneto realized his entire world should never have existed, and resolved to change history once again.
The story was relentlessly bleak, demonstrating the full horror of Apocalypse's reign. The "Alpha" issue that kicked things off set the tone, with Bishop clambering up a mountain of bodies. From there, creative teams were given a phenomenal license to rewrite the various X-Men books, with each showing the path various heroes' lives - and, indeed, the world itself - would have taken if not for Charles Xavier. The "new" titles were:
- Astonishing X-Men was replaced Uncanny X-Men, with Rogue leading a team against Apocalypse's son, Holocaust.
- Amazing X-Men in place of X-Men, with the Horsemen hunting Apocalypse.
- X-Calibre in place of Excalibur, which saw Nightcrawler travel to the Savage Land.
- Gambit and the X-Ternals replaced X-Force, with Gambit heading to the Shi'ar Galaxy
- Generation Next in place of Generation X, in which Colossus and Kitty Pryde led the next generation of X-Men on a suicide mission.
- Weapon X substituted for Wolverine, with Logan and his lover Jean Grey working for the Human High Council based in Europe.
- Factor X replaced X-Factor, starring Cyclops and Havok as they competed for Apocalypse's favor.
- X-Man in place of Cable, introducing a genetically-engineered "son" of Cyclops and Jean Grey.
It helps that the X-Men comics were at their height, and as a result Marvel had some of their best creative teams - including some remarkable artists - working on the character designs. The trick was to put each and every hero through the wringer, but also ensure they were still recognizable. Relationships were switched up, based in part on popular arcs; the Cyclops/Jean Grey/Wolverine love triangle was twisted around, the sibling rivalry between Cyclops and Havok took a brutally competitive twist, and Magneto and Rogue were married with a child.
"Age of Apocalypse" became one of the biggest successes in the history of Marvel Comics. It was a "What If?" story on an unprecedented scale, and the structure - an Alpha and Omega issue, with miniseries charting the different narratives - was tremendously effective. In truth, this is a story Marvel has never equaled - not in the various What If? miniseries, and rarely in the entire X-Men range.