After Secret Wars essentially reset the Marvel Universe, the new era brought a lot on unanswered questions with it. In the realm of the X-Men, one of the biggest mysteries of the era was what happened to Cyclops. At the start of this era, the mutant leader was revealed to be dead, and the circumstances surrounding his death made him a hero to some and a villain to others.
Another one of the era's biggest questions revolved around the problem of the Terrigen Mist. Although it activates the superpowers of latent Inhumans, it gave mutants a lethal case of the M-Pox. However, in 2016, Marvel would reveal the answers to both of these questions with the Death of X miniseries.
WHAT IS DEATH OF X?
Before Death of X, Cyclops had become a mutant revolutionary, swiftly seeking justice for mutants and protecting his people. Of course, some of the X-Men didn't agree with his methods after events like Schism and Avengers vs. X-Men, but others were firmly placed in the "Cyclops was Right" category.
Despite this ideology difference, he wasn't a villain --he was a hero although one that veered towards more antihero than the straight arrow, boy scout demeanor that had defined him previously.The events leading up to Death of X suggested that Cyclops had done something unforgivable that changed the course of mutantkind forever. However, the truth wasn't quite so simple.
Death of X told the story of how Cyclops spent his final days dealing with the emerging Terrigen Mist problem. When Cyclops and his band of revolutionaries arrived on Muir Isle to answer a distress call, he found Jamie Madrox, their old teammate Multiple Man, and dozens of his dead copies. With no regard for his own life, Scott Summers rushed down to help them.
Jamie had been poisoned with M-Pox by the Terrigen Mist cloud, leaving him gasping for breath and eventually dying in Scott's arms. The event had shaken the usually stoic leader, and he vowed to rid mutantkind of the Terrigen cloud. However, Scott was infected with the Terrigen Mist and shortly after, he died in the arms of Emma Frost. His last words were to make sure she didn't let their people end up like he did, pleading with her "don't let it end."
EMMA FROST'S RUSE
For most of the past 15 years, Emma Frost, the X-Men's one-time villain, had been in love with Scott Summers, sticking beside him throughout even the stickiest situations.
As Emma herself put it in Death of X, Scott Summers was "the only man I ever wanted to give a damn about me." Naturally, his death shook her, cradling her one great love in her arms as he took his last breath. Emma thought Scott deserved a better death than the one he got, a cold, lonely and insignificant death by M-Pox.
To Emma, Scott had spent his whole life fighting for mutant rights, and she wanted to give him a death that had greater meaning to that cause. Using her immense telepathic abilities, she fabricated a story about Scott tracking down the Inhumans to hold them accountable for their crimes and killing mutants with their Terrigen Mist.
This story takes up the first 3 issues of Death of X, where Scott valiantly charges up to Black Bolt and makes his final stand, seemingly dying in the process. By the end of the series, Cyclops' brother Havok confronted Emma about Scott's apparent demise, not believing how his final act played out.
After she told him the truth, Emma confesses that by making everyone believe in Scott's sacrifice and that he died for a cause, she was able to make him live on forever through his legacy. "I made him an idea...and ideas never die," she says, desperately holding on to the memory of Scott.
A NEW X-MEN ERA
The events of Death of X retroactively explained the era that the X-Men were already living in. At the time, the Inhumans and the Terrigen Mists were the biggest problem facing the X-Men and mutants as a whole. The Beast was on Attilan trying to figure out a cure with Inhuman leaders, and the X-Men were trying to protect their own while dealing with Cyclops' complex legacy.
Eventually, Emma's ploy was revealed in the Inhumans vs. X-Men storyline, which was the culmination of all of these events. By the end of that storyline, the X-Men were rid of the Terrigen Mists, and they attempted to move on with their lives, no longer burdened by the threat of extinction. While Emma's revelation redeemed Cyclops' legacy to an extent, she descended into outright villainy for a few years until shortly after Cyclops' eventual resurrection in 2018.