Is that really how we say it? Inferno... from back in the days when every crossover event in the Marvel Universe seemed to spin out of the X-books. Nothing like today's that all spin out of the Avengers. Inferno, if I'm not even confusing it a little bit with Fall of the Mutants, had Hell coming to Earth/New York, demons in the streets, people getting corrupted, etc. etc. How this is a mutant storyline is easily explainable by two words: Chris Claremont. Because it's never enough to be just a mutant teenager, you've also got to be an immortal, ninja, half-demon, clone, and/or from the future to be a viable character.
What If vol.2 #6 (Mid November 1989)
Based on: New Mutants #73
The true history: At the end of the Inferno event, Magik sent the demon S'ym back to Limbo along with most demons, throwing her soulsword after them to close the portals and cut our dimension off from the nether realms forever. Co-conspirator N'astirh was destroyed by the mutants, while his creation, the Goblin Queen (Madelyne Prior, perfect physical copy of Jean Grey), was defeated by Marvel Girl herself.
Turning point: What if S'ym and the soulsword got to Limbo at the exact same time?
Story type: Everybody dies
Watcher's mood: Logoed
Altered history: S'ym gets his hands on the soulsword and uses it to return to Earth, just after N'astirh is destroyed. He quickly allies with Madelyne, and they defeat the X-Men, destroying Jean Grey utterly and corrupting the rest. Then they sacrifice her baby and Hell comes to Earth permanent-like.
In short order, the planet is turned into a nightmare, with most people turning into demons, and those not corrupted are used as slaves, including Alicia Masters in the above panel. S'ym and Madelyne become consorts (they both have gratuitous Y's in their names and are pure evil, more things in common than most couples, really) and keep Wolverine as their favorite baby-eating pet. A resistance of course springs up: Doctor Strange, Doctor Druid, Shaman, Jennifer Kale, Son of Satan, Dakihm, Margali Szardos, She-Hulk, Thor, Spider-Man, Human Torch, the Captain (Steve Rogers), Kitty Pryde, and... Baron Mordo? Ok then.
Then the demons find them and attack... Aww jeez. I just took the time to write down everyone's name and most of them are dead already (page 10). So is Spidey, so is Druid, even Thor. Only Strange, Mordo, Kitty and the Torch escape. While the baddies put Dr. Doom and a corrupted Reed Richards to work on extending Limbo to all planes of existence, Strange leaves yet another hide-out to find the source of immeasurable power that is the Phoenix Force, currently residing in Rachel Summers, daughter of Cyclops and Jean Grey in an alternate future, and currently in our time but turned into a storefront mannequin by demons. (Why? Because Excalibur.) Strange breaks the spell and she joins the resistance.
The demons finds them AGAIN, and it becomes clear that they've got an inside man. It ain't the Human Torch, cuz he's soon dead, so I guess it's Mordo.
Yes, I'm as surprised as you are. Madelyne and Rachel fight for control of the Phoenix Force while Strange keeps the demons at bay. Wolverine almost stabs him in the back, but Kitty gets in the way and gets a fatal clawing in return. Then Mordo switches sides again and destroys S'ym, but only to get the soulsword and take his place at Madelyne's side. His victory is short-lived:
(You know, I probably could have filled this post with pictures exactly like this.) With his dying breath, Mordo kills Wolverine, melting all his flesh away and leaving an adamantium skeleton. Madelyne gains control of the Phoenix Force, but she can't use it much because the Wolverine skeleton, animated by S'ym, runs its claws through her undercleavage.
He is revenged for his betrayal and the Force returns to Rachel. So she cleanses the world of all its demons before returning to the Living Tribunal's embrace.
What's left of humanity returns to nature, and our story ends on a Note of Hope(TM): Alicia Masters-Storm gives birth to a baby she calls Johnathan. Awww... wait, who the HELL is the father? The only male at the delivery is the doctor (Strange), and she just spent some time being some demon's slave. She can't have gotten pregnant by Johnny before Inferno, because that's after the turning point (or did she get captured shortly after becoming pregnant post-Inferno?) And anyway, hasn't history proven that this Alicia is a Skrully Skrull Skrull? In any case, the future isn't so bright I've got to wear shades.
Books canceled as a result: All of them. This is an extinction-level event for the Marvel Universe.
These things happen: The Claremont era of the X-books was filled with permutations of Hell of Earth, but nothing so drastic was ever allowed to remain in continuity. Aren't you glad?
Next week: What if Wolverine Was an Agent of SHIELD?
My guess: Lots of LMD body parts lying around.