We've had comics being set between previously released issues. We've had comics set between panels. The bulk of the Magik: Storm and Illyana miniseries is set between two parts of the same panel of Uncanny X-Men #160. In that, Illyana is lost in Limbo, and ages half a dozen years while the X-Men watch.
The experience has clearly affected her more than the X-Men suspect. Xavier noticed something odd with her on the scanner, she was honouring an absent friend's birthday (not the brood-fighting Kitty Pryde, although it was her birthday) in New Mutants #3, and then there was that incident with the Danger Room and the sword in Uncanny X-Men #171.
Illyana Nikolovna Rasputin (named here with the correct patronymic for the first time - they even retcon in a previous usage) recounts the tale of her experiences in Limbo - her friendship with an aged Ororo and an adult Kitty Pryde (calling herself Cat), their destruction at Belasco's hands, and her seeking revenge, which in itself corrupts her.
It's not a happily ever after. The Illyana who escapes is warped, and has become exactly that vision of future Illyana that Ororo and Cat had sought to prevent. She has learned all of Belasco's spells, can teleport with the aid of discs (her mutant power?), has created a Soulsword from a bit of herself, and started to grow horns and a tail. She escapes, at which point she appears with the X-Men. At a time and place of her own choosing, it seems.
This is a good story, the best thing I've read yet in the course of my reading for this blog, and it really lays out the character of Magik, who I always had difficulty grasping. By a happy coincidence, I read this the same week that Uncanny X-Men #18 dropped, which has Magik admit to trying to bring Colossus down to her level - perhaps trying to recreate the demonic Colossus she meets here in Limbo. "No snowflakes in hell", she says. She's not wrong.