Beyonder removes Magik’s dark side (“Darkchild”), the same way he removed Moondragon’s dark side (“Dragon”) in Defenders #151.
For reasons that are unclear to me, this gives Kitty the soulsword power and releases a bunch of demons from Limbo. Kitty did wield the power back in New Mutants #30, so there is a precedent for it, but if Beyonder has all-power it’s not clear to me why the soulsword has to transfer to anyone.
Anyway, they fight and, of course, Illyana realizes she has to take the powers back in the end because they’re hers to deal with. Just like literally everything Beyonder does, it gets reversed pretty quick. If he’s supposed to be God, he’s not much of one.
To quote movie Hulk, he’s a “puny God.”
Also, Sunspot bristles under Magneto’s leadership and quits the team. Bill Sienkiewicz is just the inker now, over Mary Wilshire’s pencils.