While all-new Inferno event is coming for the X-Men and the mutants of Krakoa, it's hardly the first time the X-Men have endured the hottest flames in the Marvel Universe. In the original "Inferno," Illyana Rasputin was tricked into opening the portal to Limbo, releasing demons upon the Earth. Madelyne Pryor made a demonic deal to save her son, who grew up to be the time-traveling soldier known as Cable, but lost her humanity in the process. Meanwhile, the X-Men and X-Factor came together and defeated these demonic forces from taking over the planet.
While the heroes won that hellish battle, 1989's What If...? #6 asked "What If the X-Men Had Lost Inferno?" The issue -- by Danny Fingeroth, Ron Lim, Keith Williams, and Tom Vincent -- took a hard look at the evil that would befall Earth if the mutants had failed in on eof the darkest corners of the Marvel Multiverse.
The tipping point of the original Inferno was when Jean Grey defeated Madelyne Pryor. The What If...? flipped the outcome where Madelyne killed Jean and brought Wolverine over to the dark side. Madelyne Pryor sealed her fate when she let the demon S'ym kill her baby to keep the portal to Limbo permanently open.
Hunted by the demons, Doctor Strange led a mix of mutants and Avengers to come up with a plan to save the world. Doctor Strange assembled every surviving sorcerer, good or bad, to help him overcome the evil magic. As more of the heroes perished, including She-Hulk and Thor, hope seemed to be slipping away. Strange thought of one last-ditch effort plan to call the Phoenix Force to help purge the demons.
Rachel Summers, the last human host to the Phoenix, had been frozen into a department store mannequin since the beginning of the Inferno battle. Strange freed her, hoping her essence could call the cosmic power back to Earth. However, they were betrayed by the villainous Baron Mordo. S'ym, the Goblin Queen Madelyne Pryor, and the feral Wolverine attached them as they tried to call the Phoenix. Johnny Storm, the Human Torch, sacrificed himself in a huge flame to buy Doctor Strange and Rachel more time. When Wolverine spotted a vulnerable Doctor Strange, whose focus was on keeping Rachel safe, Kitty Pryde stepped out of the shadows to distract him. Wolverine killed her before she could phase out. Seeing his once dear friend dead by his own hand, Wolverine realized how monstrous he had become and sought revenge on those who had pushed him there.
Wolverine died trying to atone for his sins. The dwindling heroes still faced a major setback when Madelyne Pryor took over the Phoenix before Rachel could. S'ym had been killed yet his demonic rage fueled him into reanimating Wolverine's adamantium skeleton and killed Madelyne Pryor. With the host dead, the Phoenix filled Rachel. Together they were able to burn the demons from the Earth.
With the job done, the Phoenix Force flew into space and encountered the Living Tribunal. The two cosmic entities came to a truce that Earth was to be left alone. They both felt that balance had been restored to the planet after the demons were cleared away. The planet was purged and returned to its original primal state. The few people that remained had to rebuild their world. The last panel showed Johnny Storm's child being born, with the help of Rachel and Doctor Strange, giving hope to humanity. The What If...? was a glimpse of what could happen if the mutants fail during "Inferno," proving why it's one of the most dangerous words Marvel's mutants can hear.