The backstory: Belasco took Illyana under his tutelage as a young girl. An X-Men rescue attempt was only semi-successful as she was pulled back into the mainstream reality having aged several years. She had spent them under demonic tutelage, tapping into evil sources to become a master a magic. As her soul reached a near-full demonic taint, her tie to Limbo was broken and inside a ghastly shell was the young Illyana with no memory of what happened. Unfortunately, her life didn't last long as she died from Stryfe's Legacy Virus.
But then "House of M" happened and Illyana was resurrected in the re-written reality as her teenage self. With an eye on all realms, Belasco discovers his pupil is back and brings her back to Limbo. Magik betrays and defeats Belasco in her demonic form and tries to build herself a new soul, ripping a piece from New X-Man Pixie.
This brings us to the events of X-Infernus. During a sparring session in the Danger Room, Pixie takes out her "soul dagger" and - like a girl possessed - stabs Nightcrawler with it. While Beast freaks out, Pixie pulls Illyana's soulsword out of Kurt. It turns out Kurt's ex-girlfriend Amanda Sefton - who had ruled Limbo for a time - had hidden the sword within him. Magik appears, takes the weapon and suddenly is human again. Colossus is pleased to see his sister, but she wants no part of him.
Now this is all well and good. Neat little twist, neat little nod to history. Too bad the rest of the story ABSOLUTELY SUCKS.
While this is happening, Belasco's daughter Witchfire has taken Illyana's amulet which has part of Pixie's soul and the souls of... I don't know. Something else. I really don't care. Colossus, Wolverine (because he has to be in this story, I guess), Nightcrawler, Rockslide, Pixie and Mercury head to Limbo to help Illyana get the amulet back. A few people get possessed, everyone ends up happy and it's all a poorly-paced, poorly-written, adequately drawn (at best) piece of garbage. To save you the time, they don't get the damn amulet back, Pixie loses more of her soul and I don't give a ****. Not a single moment from their entrance in Limbo to the exit carries even an ounce of weight. It's like CB Cebulski is saying "Here's some ****. There it is. Not organizing it for you." A monumentally lazy effort.
In truth, this whole exercise is just an excuse to bring Magik back into the fold so they could reform (most of) the original New Mutants for an ongoing title. While this was a great idea, would it really have been so hard to involve, say... ANY of them? Karma, Cannonball and Sunspot show up for a bit, but why the hell weren't they on the strike team instead of say, Wolverine? Oh, right: sales. While that's understandable, it completely misses an opportunity to help launch the damn title this whole shebang was for.
Ultimately, the return of Magik paid off (after "Schism,") but calling this a sequel to "Inferno" is an insult. The "Sisterhood" story had a more valid claim to that given it involved the elements people actually gave a damn about. This is terrible, terrible stuff and the X-Men franchise at its worst.