Illyanna has so much stuff going on with her powers and background that it can be hard to streamline!
What I'm thinking of is that she's an Alpha-class mutant (not as high as Omega class, like Legion of Proteus) reality warping mutant. Her power expresses itself as space warping, via her stepping disk teleportation.
An evil mutant with similar powers, let's say Seline (Perhaps time-based, able to keep herself and her worshippers young, or age people to dust) picks her up as an apprentice. Reality-warping powers can be used in other ways, but doing that has bad effects (Like how Legion and Proteus are both insane)... She uses sorcery as a mental discipline to keep herself together (though pushing her power still has some side effects... for her, maybe aging and needing to drain people). The ritual and mental focus is what's important, not the words and symbols in the books.
She has big plans for Illyanna, wanting to merge time and space warping and become unstoppable. So Illyanna's forced to learn from her, possibly while in a quick time pocket Selene made. When she presses her powers too much for "spells," demonic features appear on her body, the horns, hooves, tail and such.
During her forced apprenticeship, she develops a secondary mutation, like Colossus's... she's able to armor part of her body and generate an organic steel "soulsword." Since it's part of her body, she can put a stepping disk on the edge of it and cut through anything, or maybe just inorganic things. Using that, she was able to escape and go to the X-mansion. She might have been 13 when her first powers manifested and she was picked up, and 17 now, though only a few moments passed in the outside world.
Selene's still out there and wants to possess her, and she doesn't talk about what happened to her in those four years of being her "apprentice."
Would that fit?