The Uncanny X-Men #134 (1980)
By Chris Claremont, John Byrne, Terry Austin
Background: Most of the X-Men are prisoners of the Hellfire Club, except for Wolverine. Jean Grey has been turned into the Black Queen under Mistermind's control. The X-Men active roster is: Cyclops, Storm, Colossus, Wolverine, Nightcrawler, Jean Grey.
So, What Happens? Cyclops has been defeated by Mastermind in a mental duel, but Wolverine breaks into the room where the X-Men are kept prisoner, and at the very same moment Jean frees herself of Mastermind's control and unlocks Cyclops' helmet restoring his powers. In turn, Cyclops frees the rest of the team, allowing the X-Men a rematch against the inner circle of the Hellfire Club. Harry Leland, the mutant capable of increasing someone else's mass, makes a fatal mistake and is quickly killed by Wolverine. Colossus disables the cyborg Donald Pierce but can't prevent his escape. Likewise, Sebastian Shaw defeats Nightcrawler but has to run away from Storm, whose weather powers are more than he can handle. However, the worse is yet to come: as a blackout plunges the club mansion into darkness, Jean Grey feels herself consumed by a dark power but is unable to do anything about it. The first one to suffer her wrath is her former tormentor, Mastermind. After explaining how mentally killing Cyclops broke Mastermind's hold over her, Jean expands his mind into the universe, to the point where he can't cope anymore and simply shuts down. Jean then returns to the X-Men and together they flee the building, but within moments the darkness within her takes full control and she emerges as Dark Phoenix, destroying the team's skycraft with everybody on board.
