Uncanny X-Men #210, 211, 212, 213
X-Factor #10, 11, 12

The Mutant Massacre, perhaps the worst battle the X-Men ever fought, was set into motion by Sinister, who sent his Marauders to clean out the Morlock tunnels the best way they knew how; by killing them. The X-Men answered the distress call when one of the Morlocks came seeking help, badly beaten, shot, and nearly dead. The X-Men hurried to the tunnels where they engaged the Marauders in combat, but too many of the Morlocks were already dead. Saving whom they could, the X-Men fought their way out of the tunnels...but not without their own losses. Shadowcat valiantly tried to save Rogue by phasing her when Harpoon, a Marauder, launched one of his energy harpoons at her. It worked, but left Shadowcat in a state of permanent phase. Colossus suffered massive damage, so much so that he could not revert to his human form for fear of death. Nightcrawler was torn to shreds by Riptide, another Marauder, so gravely injured that he was left in a coma for several months.

Meanwhile, X-Factor had come upon the scene as well, fighting their own battle against the Marauders and saving what few Morlocks they could. Though they fought valiantly, they did not escape without their own losses. Angel was nearly killed by Harpoon when his wings were pinned to the wall and he was beaten, leaving him unconscious for days. He awoke to find his wings amputated, gangrene having been setting in.

Back at the X-Mansion, things were slowly falling apart. Three of the X-Men incapacitated, nearly all of the Morlocks dead, Storm began to doubt the wisdom of her leadership of both groups. Breaking down and running away, it was Callisto, former leader of the Morlocks, whom Storm had nearly killed to win leadership of, who brought her back to her senses. Meanwhile, Wolverine was fighting one of the greatest battles of his life in the tunnels against the Marauder Sabretooth, his old nemesis. He brought down the tunnel wall between them in order to escape, and bring the Morlock healer back to the mansion to heal those who were still clinging to life. The X-Men were scattered about the grounds when Sabretooth came to the mansion, and it was through the bravery of Psylocke that the few remaining Morlocks survived. She held him off until Wolverine returned and finished the battle, sending the villain over a cliff. It was this act of heroism which earned Psylocke a place in the team.

The casualties were many, the dead numbered many more. Few Morlocks survived. Colossus was beyond the healers ability, and it took the supreme efforts of Magneto to repair his armored form. Still it took him many months to regain control of his mutant ability. Shadowcat was beyond any help, and it took the combined efforts of Dr. Doom and Reed Richards to save her from certain death. Nightcrawler recovered with time, but it took a long time for his teleporting ability to return to normal. Recent events in the X-Men's lives have revealed that the X-Man, Gambit, was the one who brought the Marauders together under Sinister's guidance. He did not know the full extent of the horror which would follow, and even tried to stop the Marauders from causing the bloodbath, nearly getting himself killed in the process. He managed to save one child, the girl who grow up to become an X-Man as well, Marrow. Though he had put that part of his life behind him, the X-Men disowned him, the memories of this particular battle to bitter for them to bear his presence. Sinister did not achieve his ultimate goal of destroying the Morlocks altogether, but the effects of the Mutant Massacre are still being felt today, and will never be forgotten.