#12. X-Men vs. the Shi'Ar Imperial Guard
As mentioned in the fight between the X-Men and the Hellfire Club (which was handled by the same creative team as this battle – Chris Claremont and John Byrne), the end result was that Phoenix had her mind messed with so much that she eventually snapped and basically became a separate entity, calling itself Dark Phoenix.
The X-Men battle her in “Uncanny X-Men” #135, but she quickly defeats them and flies off into outer space. Her traveling makes her yearn for sustenance, which she gets by entering and imploding a star, soaking in the energy of its destruction. She does not care that the destruction of the star also destroys the planet it orbits. A starship of the Shi'Ar Empire notices, though, and challenges Dark Phoenix.
She destroys the ship easily, but not before it gets off a message to the Shi'Ar Royal Throneworld, where the Empress of the Shi'Ar Empire, Lilandra (Professor X's current lover) springs into action.
Meanwhile, in “Uncanny X-Men” #136, Dark Phoenix returns to Earth where her teammates and her love, Cyclops, await her with a device meant to shut down telepaths. She destroys it and once again takes care of her teammates with ease, but Cyclops manages to calm her down by appealing to her still human side. At this point, Professor X attacks, and he and Phoenix have a telepathic battle, where ultimately, due to the aid of whatever vestiges of Jean Grey remain in Dark Phoenix, he manages to shut Dark Phoenix's powers down.
The X-Men do not have a moment to rest, though, as they're instantly teleported to a Shi'Ar battleship orbiting Earth, where the Shi'Ar Imperial Guard and Empress Lilandra demand Jean Grey be delivered over to them for punishment for her actions as Dark Phoenix. Professor X utters a Shi'Ar ritual challenge, which Lilandra is duty-bound to accept. Therefore, in “Uncanny X-Men” #137, the X-Men will fight the mighty Shi'Ar Imperial Guard for the fate of Jean Grey.
The next day, the teams meet on the Moon for their battle. The X-Men are heavily outnumbered and outclassed by the Guard, who are made up of the most powerful heroes of the Shi'Ar Empire. Although the X-Men fight valiantly, they are slowly picked off, one by one, until only Cyclops and Jean remain free. When Cyclops is taken out as well, Jean begins to panic and the limits Professor X placed on her begin to crumble – Dark Phoenix frees herself and wants revenge. The X-Men stand ready to battle Dark Phoenix, but Jean manages to take control long enough to intentionally trip a defense mechanism laser, killing herself so that Dark Phoenix can hurt no one else ever again.