The Dark Phoenix Saga is an extended X-Men storyline focusing on Jean Grey, and the Phoenix Force, ending in Jean's apparent death. It was written by Chris Claremont with art by Dave Cockrum and John Byrne.
It is sometimes divided into two parts, with the Phoenix Saga (X-Men Vol. 1 101-108, 1976-1977) referring to Grey's seeming assumption of the power and repair of the M'Kraan Crystal, and the Dark Phoenix Saga (X-Men Vol. 1 129-138, 1980) to her corruption and fall. It is one of the most well-known and heavily referenced comic stories, and widely considered a classic.
In comic books today, readers know the Phoenix Force as a psionic cosmic entity linked to Jean Grey. However, this was not how the character was written in the original story - there, the Phoenix actually was Jean, at the very peak of her power. Returning from a mission in space, the story told of Jean being exposed to the deadly radiation of a solar flare, and briefly attaining her ultimate potential as a telepath and telekinetic. In this moment, Jean became a being of pure thought, and then reformed herself upon return to Earth with the new costume, identity and power of "Phoenix." It was with this incredible power that Jean repaired the fractured M'Kraan Crystal, but in doing so, opened the doorway to greater, more dangerous sensations.
Jean's increased powers began to corrupt her, leaving her with an ever-increasing desire for more sensations that eventually separated her consciousness into the Jean the X-Men knew and loved, and the decadent Phoenix, which Jean just managed to keep in check. This hunger made her an easy target for the illusionist Mastermind, who was attempting to prove himself in order to join the prestigious Inner Circle of the Hellfire Club. With the help of a mind-tap device created by the White Queen, Emma Frost, Mastermind (using the alias Jason Wyngarde) was able to project his illusions directly into Phoenix's mind. Believing herself to be his 18th Century lover, Lady Grey, Phoenix was subverted into joining the Hellfire Club as their Black Queen, a decadent role that would allow her to relish the extremes of human emotion.
When the X-Men came to her rescue, they were captured by the Inner Circle, and Jean's true love Cyclops faced Mastermind in a psychic duel. When Mastermind killed Cyclops' psychic image, it served to break his hold over Jean's psyche and unleashed the insane power of the Phoenix, which consumed Jean utterly, and renamed itself the "Dark Phoenix". The X-Men battled her in vain, but were easily defeated by her power before she departed for the heavens. Intent on satiating her hunger, Dark Phoenix created a wormhole and transported herself to a distant galaxy. Without a thought of the consequences, she dove into the heart of the D'Bari star and devoured its energy, causing the star to go nova — killing billions of innocent alien lives in the process. Dark Phoenix was then attacked by a Shi'ar vessel to prevent her from destroying other stars. Dark Phoenix easily defeated her foes, but not before they were able to alert the Shi'ar Empress Lilandra. Gathering a host of intergalactic associates, including the Kree and Skrull empires, the council concluded that Dark Phoenix was an even more serious threat than the planet-consuming Galactus and must be destroyed.
On Earth, the X-Men were greeted by Avengers member, and former member of the X-Men, the Beast. He was able to design a device which would neutralize Phoenix's powers long enough for them to defeat her. Dark Phoenix returned to Earth, to her family's home, and was subsequently attacked by the X-Men. During a vicious psionic battle with her mentor, Charles Xavier, he was able to rebuild the psychic 'circuit-breakers' in her mind which reduced Dark Phoenix's powers to more reasonable levels and allowed Jean's personality to reassert control, curtailing the destructive impulses of Dark Phoenix.
The Shi'ar then abducted the X-Men, told them of Dark Phoenix's casual genocide, and indicated that she must be put to death because of it. Xavier, who was romantically involved with the Shi'ar Empress, challenged Lilandra to Arin'n Haelar, a Shi'ar duel of honor that cannot be refused. After conferring with her allies, who insisted the contest be staged to ensure a guaranteed victory on their part, Lilandra ceded to Xavier's demand.
The next day, the X-Men and the Shi'ar Imperial Guard were teleported to the Blue Area of the Moon of the moon where they would do battle, with the victors deciding the fate of Phoenix. The Imperial Guard, led by Gladiator, was able to defeat all of the X-Men, leaving Cyclops and Phoenix alone to make a final stand against them. When a stray bolt of energy hit Cyclops, Jean Grey's panic overrode the psychic circuit-breakers Xavier had placed within her mind and the full might of Phoenix's powers were once more unleashed. At this point, Lilandra abandoned the delicate approach and ordered Plan Omega, which would consist of destroying the whole Solar system in hopes of eliminating Dark Phoenix in the process.
With events spiralling out of control, Xavier ordered the X-Men to subdue Jean to preempt Lilandra's emergency measure. The team battled her until she regained her senses. Running to a back alley on the moon, Jean, struggling to keep control, activated a Kree weapon and disintegrated herself after an emotional good-bye to Cyclops. He later deduced that Jean had planned her sacrifice from the moment they had landed on the moon.
This pivotal story ends with Uatu the Watcher commenting that "Jean Grey could have lived to become a god, but it was more important to her that she died as a human."