While it is a part of comic book reality nowadays, back in the late '80s there had never been a crossover between the popular X-Men related comic books. In fact, until the early 80s, there was only one X-Men title, "Uncanny X-Men!" But by 1986, there was the regular "X-Men" title, there was "New Mutants" (detailing the next generation of mutant heroes) and "X-Factor" (starring the original five members of the X-Men), and in the fall of 1986, the first X-Crossover took place detailing the "Mutant Massacre."
The Mutant Massacre featured the Marauders, a team of vicious killers employed by the newly introduced X-Men villain Mr. Sinister, going into the New York sewers, where a community of mutants known as the Morlocks lived (the Morlocks were mutants who tended to be disfigured or were otherwise unable to fit in living with "normal" humans). At this point, the Marauders proceeded to murder as many Morlocks as they could. The X-Men entered the tunnels to save the Morlocks, and engaged in a dramatic and deadly battle that lasted from Uncanny X-Men #211 to #213 (all three issues were written by Chris Claremont, with John Romita Jr. drawing the first issue, Rick Leonardi the second and Alan Davis the third).
The X-Men suffered critical injuries soon after entering the battle, when the teleporting X-Man Nightcrawler, who was recovering from a recent injury and had only recently regained the ability to teleport, used his powers to disable one of the Marauders. However, he was unable to use his powers once he was finished, leaving himself vulnerable to the Marauder Riptide, a mutant whose power involves sending barrages of razor sharp blades flying people at high speeds. Nightcrawler was severely injured by Riptide.
This led to one of the most dramatic moments of the war when the X-Man Colossus determined that the only way to stop Riptide was to use deadly force. As Riptide continued to pummel the X-Man's metal body with blades, Colossus forged forward until he was able to snap Riptide's neck.
At this point, Colossus collapsed due to the wounds he incurred during his fight. As it turned out, he was so injured that while he could survive in his metal form, he could not transform back to his human form. Meanwhile, the X-Men suffered another casualty when Kitty Pryde was injured and trapped in her intangible form.
While the X-Men return to their home to recover with the Morlocks they manage to save, the deadliest of the Marauders, the evil Sabretooth, makes his way to the X-Men's home. During the course of his journey, Sabretooth tangled both with Wolverine and ultimately with the telepathic Psylocke, who was staying with the X-Men at the time.
In the end, the X-Men managed to save many Morlocks (X-Factor also saved some, in a separate excursion into the Morlock tunnels), but the team was forever changed, with longstanding members Kitty Pryde and Nightcrawler leaving the team and new members like Psylocke joining the group. The most important change for the team was that they no longer had any illusions of safety at their home, and soon left the X-Mansion entirely.