"Magik: Storm and Illyana" explains what happened to Illyana Rasputin in the panels between when the X-Men escaped Limbo in "Uncanny X-Men 160" and when they pulled a suddenly-older Illyana through a portal behind them. It wasn't until later that the X-Men learned of Illyana's new powers of sorcery. This story, released a year-and-a-half after the Limbo issue, explains it all.
"Uncanny X-Men 160" itself was warped to begin with and a real narrative departure for the X-Men in the vein of the excellent "Nightcrawler's Inferno." The X-Men came face-to-face with either dead versions of themselves, or twisted recreations. This is the world Illyana is trapped in, with a more cat-like Kitty Pryde, an evil Nightcrawler and an old, frayed, magically-skilled Storm. Demonic ruler Belasco wishes to corrupt Illyana, creating an heir to his realm. The girl is easily corrupted from the start, and is whisked away by Storm before matters get worse.
Despite Storm's efforts to train Magik to use her newfound magical powers for good, everything in Limbo has a root evil and her work seems to inevitably draw closer to a losing effort. It's with this thought that a grown Kitty - showing cats' eyes and fangs - forces Illyana to leave and trains her in traditional combat. They storm Belasco's castle, and things go horribly. Illyana's tutelage under Belasco continues unabated, and the drawing climax no longer seems to be about whether the girl will become evil: it's a question of HOW evil she'll be.
This matches up fairly well to Claremont's other works, as he's always been skilled at creating tension and character crises. While it's certainly not his best mini-series ("Wolverine" with Frank Miller. Duhhhhh,) I'd rank it fairly well against say... "Wolverine and Kitty Pryde." Claremont writes young women well, and this is no exception.
And then there's the art. John Buscema draws the first two issues with Ron Frenz and Sal Buscema imitating him for the final two. Tom Palmer inks all four. As I said in my Giant Size review covering most of Roger Stern's run on "Avengers," I was not impressed with their work on that title, considering it in-house mediocrity. This... is better. The setting allows more freedom for creative backdrops, and the strange melting faces fit into a twisted realm like Limbo. Definitely bottom-half in terms of art in a Claremont book (the guy DID work with John Byrne, Frank Miller, Marc Silvestri, Bill Sienkiewicz and Jim Lee, after all,) but it's acceptable.
"Magik: Storm and Illyana" is a nice little self-contained story. There's better Claremont X-Men stuff out there, but you'd be hard-pressed to find anything he did with that franchise in his first go-around that wasn't at least "very good." This fits that model.