The original Mutant Massacre (capital letters, as opposed to any other time a big number of mutants died, like when Sentinels wiped out 16 million mutants on the island of Genosha) was when Mr. Sinister hired a bunch of mutant mercenaries, known as the Marauders, to wipe out the Morlocks - the mutants too weird or just plain ugly to live with the 'normal-looking' people - living in the sewers and tunnels underneath the city. We never found out how many actually died, but it's implied to be hundreds, they were supposed to be a big community.
Biggest fallout of the storyline was that Angel wound up losing his wings, having to get them amputated. That led to him being picked up by Apocalypse and turned into one of his Horsemen, a plot point that still crops up today - Apocalypse's hold on Angel is considered to be one of his defining character moments/traits now, though I haven't seen much with him in the current Krakoan era, and and Big A is considered an 'ally' right now, so I'm not sure how that plays out in the big picture.
Other interesting tidbits: Kitty got stuck in her phased form and started losing cohesion, nearly dissipating into nothingness. She was saved by Dr. Doom, who basically did it because Reed Richards couldn't and wanted to remind him who the better scientist was, before ordering everyone to duck because he was about to turn left and didn't want to knock everyone out with his gigantic dick. It took 16 Doombots to carry his testicles out of the room. Legend has it, Sue still thinks of it to this day.
Colossus wound up unable to walk and stuck in his armored form for a while, due to injuries sustained during the fight. He had to kill one of the Marauders during the fight as well, which haunted him for a long while. Nightcrawler ended up in a coma, due to the battle and injuries he already had fighting Nimrod, an adaptive Sentinel from an alternate future (seriously, 80s and 90s X-Men was wild). And Psylocke was made an official member of the team, thanks to her efforts in single-handedly leading Sabertooth away from the injured and finishing the job he started with the Marauders - at this point, there was something like a hundred injured and dying Morlocks recovering in the mansion and in Magik's corner of Limbo, basically all the survivors of the attack, and with most of the team injured, missing or otherwise out of action, he could've easily done it.
And in the 90s, Gambit, who had been hinted at as having something indescribably dark in his past, was revealed to have been hired by Sinister as the guy who put the whole thing together. That went down poorly with the rest of the team, he got booted from the squad, Angel and Wolverine nearly murdered him IIRC and it's only in recent years that other characters have begun to get over the betrayal. Either way, it's still something that gets brought up from time to time as a reason not to trust him.
So yeah, there's a reason why it's remembered as one of the big storylines from the 80s, it shook up the status quo in a big way for years and in ways that still matter even today
