Retconning

Phoenix used to actually be Jean herself at the peak of her power. Marvel decided to retcon Phoenix and make it a separate entity in order to keep Jean innocent from killing billions of life forms and to bring her back to life.

The road to Dark Phoenix

With Jean’s power increasing it began to corrupt her and made her more hungry for energy. This caused Jean to develop the split-personality but Jean learned how to keep in under control. Mastermind saw that Jean can be easily manipulated with her power hunger. Mastermind wanted to join the Hellfire Club and so he proceeds to capture Phoenix and project illusions to make her think she was in the 18th century and became the “Black Queen”.

The X-Men were captured in an attempt to save Jean. Mastermind battles Cyclops telepathically and kills his psychic image. This made Jean insane and unleashed all of Phoenix’s power. She renamed herself Dark Phoenix. The X-Men try to battle her but were unsuccessful. Dark Phoenix then proceeds to create a black hole and teleports to another galaxy. She dove into the D’Bari star to feed off its energy and caused it to go Nova.

The Shi’ar try to attack Dark Phoenix but were defeated easily. Meanwhile on Earth, Beast develops a machine that can neutralize Phoenix’s power for a short period of time. Xavier battles Dark Phoenix telepathically and Xavier was able to put the psychic blocks back in her mind and Jean would regain control.

The Shi’ar took the X-Men aboard their ship and accused Dark Phoenix of genocide and the only penalty was death. They then agree to battle for Jean’s survival. They were then teleported to a moon and fight the Imperial Guard and the Gladiator. They defeat the X-Men except for Jean and Cyclops. Cyclops was hit and this caused Jean to panic and become the Dark Phoenix. The X-Men subdue the Phoenix and she would commit suicide right after.