[Rausch]

Originally Posted by el borracho

I'm not familiar with anything past about 1986. What happened in the crossover? More mutants drug into limbo?

Super short version is limbo opens a hole to earth slowly "infecting" it. It hit all the Marvel titles in some way but mostly the X-Books. It was the first darker story I'd read from Marvel.

Two demons from Limbo, S'ym and N'astirh, plan a demonic invasion of Earth. Their plan revolves around Illyana Rasputin of the New Mutants, as her mutant power allows her to open passages between Limbo and Earth. During one of the New Mutants' routine stopovers in Limbo, N'astirh casts a spell blocking Illyana's teleportation power, thus trapping the New Mutants in Limbo with S'ym, who has taken control of Limbo's hordes and is eager to kill the New Mutants in order to solidify his claim to Limbo. Illyana assumes that the entrapment spell was cast by S'ym, and so sees no reason to distrust N'astirh when he advises her that she can return to Earth by embracing her demonic power. She does so and opens a gateway to Manhattan. N'astirh had kidnapped Wiz Kid of the X-Terminators and coerced him into building a spell-casting computer; once Illyana opened the gateway, he uses this computer to cast a spell holding it open.

The city of Manhattan falls under siege, and the Avengers, Fantastic Four, Daredevil, Power Pack, and Spider-Man fend off numerous demons, as well as Hobgoblin, now possessed by a demon, and the mutant-hating Boogeyman, transformed into a monster by N'astirh. Inanimate objects become demonically possessed and begin attacking and devouring people. As shown in Daredevil and X-Men, most residents of Manhattan treat the demonic invasion as a part of normal life in the city. Buses still run, under an all-volunteer force since the drivers had either been eaten or transformed into demons themselves. Subways function, and people ride them willingly, even though some only go into Hell. Stores still sell products. Helicopter tours run.

Meanwhile, N'astirh had made a bargain with Madelyne Pryor, agreeing to locate her son Nathan and manipulate the X-Men into killing the Marauders in exchange for her casting a spell that would make a permanent bridge between Earth and Limbo. To keep his end, N'astirh alters the X-Men's computer systems so that they can use them to locate the Marauders. Driven to bloodthirstiness by N'astirh's Inferno spell, the X-Men attack the Marauders' headquarters, gleefully killing most of them in the ensuing battle. Colossus remains unaffected by this spell, due to the protection of his organic steel armor. However, when he learns what happened to his sister Illyana, he concludes that he can only free his fellow X-Men from Inferno's influence by saving her. In fulfillment of his other half of the bargain, N'astirh liberates Nathan from Mister Sinister's laboratory, where Madelyne learns that she is in fact a clone of Jean Grey created by Sinister.

With their plan fulfilled, N'astirh and S'ym begin fighting each other for leadership of Limbo's hordes (and by extension, rule of both Limbo and Earth). With S'ym gaining the upper hand, N'astirh makes a desperate bid for victory by letting himself be infected with the Transmode Virus. He then merges with Wiz Kid's spell-casting computer, increasing his magical powers exponentially and thus allowing him to make a permanent bridge between Limbo and Earth without Madelyne Pryor's help. However, Wiz Kid destroys the computer before N'astirh can make use of this power. The explosion reduces N'astirh to ashes, but he is immediately reconstituted by the Transmode Virus.

Finding Illyana, Colossus is horrified to see that she has so completely given in to her demonic side that she is totally covered by her eldritch armor and has demonic horns, legs, and a tail. Ashamed at her brother's reaction, she flees into Limbo, and decides to end the demonic invasion by assuming rule of Limbo. However, her teammate Rahne Sinclair persuades her against this, and she instead gives up her demonic powers by creating a massive stepping disc that banishes most of the demons back to Limbo, including S'ym, then throwing her Soulsword in after them to seal the portal shut. Afterwards the New Mutants find a seven-year-old Illyana inside the husk of her eldritch armor.