Mr Sinister orchestrates a massacre of the Morlocks, a community of mutants living in the sewer system of New York. The storyline was unique for a few reasons, namely:
- It was the first mention of Mr Sinister, a character who would greatly affect the X-men.
- It was one of the first major in-company crossovers, as not only were the X-men involved, but they were being led by a Magneto who chosen to atone for his crimes. The New Mutants were also involved. As well as that, the original 5 X-men (Cyclops, Jean, Angel, Beast and Iceman) reunited to form X-Factor, who in something of a con, posed as human mutant hunters for the government, where they were help mutants where they could. Also involved was Thor and the sibling team the Power Pack.
- The storyline left several heroic mutants with life threatening injuries. Colossus was left paralysed if he shifted to his unarmoured form. Nightcrawler was in a coma. Kitty was unable to get out of her phased form, meaning she was effectively fading away. And most famously, Angel's wings were pinned to a wall, leading to gangrene setting in and necessitating the medical amputation of his wings. This would lead to his joining Apocalypse for a time.
Basically the storyline was about an end of innocence, as many younger heroes were having to come to grips with the deaths of so many innocent people, as well as older heroes having to make darker choices that they normally wouldn't make. Also Storm has a freak-out and leaves the team for a short time.
