While “Mutant Massacre” is preceded by both DC’s reality-altering Crisis on Infinite Earths and Marvel’s superhero smashup Secret Wars, both of those stories are relatively self-contained. Crisis has its “red sky” tie-ins, while various Marvel titles spent a few panels an issue teasing the arrival of Secret Wars‘ Beyonder and showing the heroes disappearing from Earth, but readers then and now can simply read the entirety of those two series, ignoring the loose tie-ins, and still come away with the full story. “Mutant Massacre”, on the other hand, unfolds across multiple titles, with events in one issue directly referenced in another and specific narrative threads passed from one title to another forcing readers to buy all the various issues to get the full story (which, then and now, remains the true purpose behind crossover event stories, of course).
The plot of “Mutant Massacre” is the brainchild of longtime X-Men (and New Mutants) writer Chris Claremont, further developed and shepherded along by his former editor-turned-franchise-partner, Louise Simonson (who was the writer of spin-off series X-Factor as well as Power Pack, and the wife of Thor writer Walt Simonson). Daredevil‘s tie-in issue, meanwhile, was written by Claremont’s then-current editor, Ann Nocenti, creating a sense that the crossover was very much a family affair, so to speak). It involves, as the title suggests, the massacre of the Morlocks, a group of mutants loosely allied with the X-Men who lived in massive tunnels beneath Manhattan because their mutant abilities left them unable to blend in with normal society, by another group of mutants, the vicious mercenary Marauders, appearing for the first time in this storyline. As the crossover unfolds, all the readers (and the characters) know about the Marauders is that they are savagely wiping out the Morlocks, as most of the story is concerned with their initial attack and its immediate aftermath (the story begins essentially in media res, with the Marauders launching their assault on the Morlocks).
In-universe, Claremont never really got around to exploring the motivation behind the Marauders’ attack (he does eventually reveal the order came from their leader, Mr. Sinister, and later writers tackle the question of why he ordered the massacre, to mixed results), but the real world genesis of the plot is actually fairly well documented, and it is born from two different circumstances outside Claremont’s control. First, when the Morlocks were initially in Uncanny X-Men #169, Claremont intended for the group to be relatively small, but then-series artist Paul Smith went wild, drawing in dozens of Morlocks to create the impression of a thriving underground community, with numerous members beyond the ones highlighted in the story.
The end result is still a storyline with a major impact on not only the X-Men but the rest of the various X-Men-related characters of the time: the Morlocks still got massacred, the surviving population a number much more in line with Claremont’s initial expectations for the group, the X-Men found themselves forced to strike back at the Marauders with deadly force in the face of their savagery, Wolverine encountered his eventual archenemy, Sabretooth, for the first time on-panel, and fan-favorites Colossus, Nightcrawler and Kitty Pryde were grievously wounded and written out of the book, opening the door for new members like Psylocke, Dazzler and Longshot, a shakeup of the series’ core cast on a level not seen seen since the “All New X-Men” launched in 1975 (Colossus returned to the team relatively soon, but Nightcrawler and Kitty were eventually spun off into Excalibur, just as in Claremont’s original plans for the storyline, and didn’t return to the X-Men until the late 90s). In New Mutants, the massacre launched the title characters into a multi-part storyline that found them lost in space and time, away from Earth for an extended period and briefly reunited with Professor X (who was trapped off Earth at the time), while in X-Factor, Angel, one of the original X-Men, was savagely attacked by a group of Marauders during the massacre, his wings damaged to the point that they were ultimately amputated, starting him on the path towards becoming the metallic-winged Archangel, one of the hallmark X-characters of the 90s.
But the impact of “Mutant Massacre” on superhero comics as a whole is even greater. By taking the pre-existing idea of inter-title crossovers in a shared universe and expanding it to include, not just two titles or a group of titles within a “family” of books (like X-Men, X-Factor and New Mutants) but titles which rarely interacted with one another, and by making each issue an important chapter in the overall narrative, thus forcing readers to buy every issue of the crossover in order to get the whole story, Marvel found a formula for sales success. After “Mutant Massacre” hit big, it was followed up a year later by “Fall of the Mutants”, another X-centric storyline that drew in such disparate series as Captain America and Incredible Hulk. Another year later, the next crossover event was “Inferno”, which expanded even further to include nearly every series set within the Marvel Universe to one degree or another. By then, the annual crossover event series was a fixture at both Marvel and DC, eventually applied to things like the yearly “annual” issues (in which each series’ annual would contain one chapter of a larger story) and expanded to absorb the standalone limited series (like Crisis or Secret Wars), in which the main narrative of the story unfolded in its own series alongside the chapters published in the regular, ongoing books. Thus, a clear line can be drawn all the way from “Mutant Massacre”, a storyline never intended to be a crossover, born out of Chris Claremont’s need to change up his intended plans mid-story, to the current event-driven climate at both Marvel and DC. The impact of “Mutant Massacre” on the X-books of the time was big. Its impact on comics was even bigger.