This is from X-Factor #10
"Mutant Massacre" was a major Marvel Comics crossover storyline, which took place during late 1986. It primarily involved the superhero teams the X-Men, X-Factor and the New Mutants. Power Pack, Thor, and Daredevil crossed over for an issue in their own comic books. The Marauders are mutant assassins employed by the X-Men archfiend Mister Sinister, with the purpose of assassination of other mutants, and act in unison as a commando strike-force to carry out acts of mass murder.
One of the facts that made this story so cool: The Marauders simply entered doing their job: killing, without explanations. A big carnage. One of the most violents histories involving the mutants . Only after the incident the things became clear and the X-men discover that Mr Sinister is the Boss of the Marauders.