The "Phoenix Force" is an immensely-powerful force of energy that inhabits the body of Jean Grey and serves as her stronger, albeit dormant, secondary personality.
Prior to X-Men (film)
When a young teen Jean is found by Charles Xavier and Erik Lehnsherr, she has unlimited access to her power, able to levitate all the cars on her block without even trying. Charles senses a dark presence in her mind and puts psychic blocks to prevent her from fully unlocking her powers.
Jean in her mid-thirties, begins to have dark premonitions and begins to lose control of her abilities. At a school field trip to the museum, Jean becomes overwhelmed by all the voices she's hearing telepathically and she begins to affect the electronics in the building. Scott Summers embraces her, telling her he's been seeing a change in her lately ever since the X-Men fought Magneto. She tells him she feels like something bad is going to happen very soon.
When the X-Jet is being attacked by military jets, the Phoenix Force briefly takes over her and detonates a missile. Days later at Alkali Lake, Jean is fighting a mind controlled Cyclops he's blasting her with his optic beams and Jean blocks it with her telekinesis. Cyclops begins to overpower her until a dark force awakens in her and she unleashes a massive telekinetic force that knocks Cyclops away, but also ricochets his energy blast creating cracks in the dam.
The X-Men are escaping Alkali Lake and enter the Blackbird, but the dam bursts threatening all their lives. Jean sacrifices herself to save them. The Phoenix Force awakens in her, giving her enough power to hold back a massive wave of water, giving the Storm enough time to fly the jet to safety. The wave then collapses on Jean, but she's wrapped in a protective cocoon of telekinetic energy. The kind and good part of Jean is buried in her mind and the Dark Phoenix part takes hold.
Scott, still heartbroken by Jean's death, returns to the Alkali Lake, where he starts to hear her voice in his mind. Shaken by this, he releases a blast of energy in the water, breaking the cocoon. It forms a vortex in the water which explodes. The Lake is dried out and Jean appears out of light. Unknown to Scott, it is not Jean but the Phoenix, awakened after too long. It contains his mutation so she can see his eyes and the two kiss before she kills him.
In the wake of his death, Professor Xavier send both Storm and Wolverine to find him in the Alkali Lake, where they find nothing but his glasses and a sleeping Jean. They bring her to the School, where Charles explains them about the nature of Jean's mutation and the Phoenix, which is now free as the psychic barriers he built are broken. Eventually, the Phoenix awakes and tries to have sex with Wolverine, who notes this behavior to be not Jean's and tries to calm her down. Jean starts to lose control over her powers before Logan states that she is not well and the Professor can help her, angering the Phoenix, who throws him away, destroys the room and leaves the Mansion.
Both Magneto and Professor Xavier get into her old home in order to meet with her and try to recruit her to their sides. Professor believes the Phoenix must be contained before it finally takes over Jean, but Magneto wishes to use her as a weapon in his purpose. The three meet and the Professor tries to imprison the Phoenix again, enraging her. The Phoenix immobilizes them both and, in hatred, telekinetically destroys the house and makes everything and everyone inside it's area to float several feet above the ground while a battle between the Brotherhood against Storm and Wolverine happens. Losing all control, the Phoenix destroys the Professor in a sub-atomical level and breaks the house. Confused by everything, she goes on with Magneto.
He tries to lure her into using her powers to help the Brotherhood destroy the Cure, but the Phoenix threatens to cure him. Wolverine tries to find Jean and reason with her but Magneto does not allow this. Later, she is present with the Brotherhood when they invade Alcatraz, but she does nothing at all, not moving a single finger to help the Brotherhood in the battle against the soldiers, and is seemingly bored with the entire event.
Eventually, Magneto is cured by the efforts of Wolverine and Beast. Logan tries to reason with her and almost gets her to return by their side when the soldiers attack, enraging the Phoenix. She disintegrates the bullets and the soldiers, and starts to do a rampage. Her limitless powers destroy the entire island, mass-murdering soldiers and mutants alike by destroying their bodies, and completely devastating the island to the point even the water around the island started to be destroyed. Logan approaches her, seeing the Phoenix delighted with the destruction she is causing. She asks if he would die for them, and he answers he would die for her. Jean speaks one last time, begging for him to save her, and Logan kills her, killing the Dark Phoenix and ending the rampage.
Prior to Apocalypse
In her late teens, Jean began to have dark and uncontrollable flashes of power, and she was sent to Charles Xavier's school. She worked with Charles and Hank McCoy, but she soon gained a bad reputation and many students began to fear and avoid her because of her uncontrollable powers.
After Apocalypse's awakening, Jean started to have visions of an incoming destruction and slaughter, which were leaving her afraid, and, with this, scaring the other students. Charles tries to calm her down, but to little avail.
During the last battle against Apocalypse, despite everybody's best efforts, they are all defeated by Apocalypse whose powers are seemingly limitless. Professor Xavier tries to fight Apocalypse in the Astral Plane but his efforts are useless as Apocalypse also defeats him. With the world seemingly fated to fall before him once again, Charles calls for Jean and begs her to unleash her powers and "let go".
Jean, initially distraught into fully releasing her powers, eventually remains as the last hope of defeating Apocalypse, so she ultimately let go and releases the Phoenix Force. The Phoenix, now free, envelops her as a massive aura of fire in the shape of a bird. The Phoenix destroys the Astral Plane, making short work of Apocalypse there, and proceeds to attack him in the real world. She destroys his armor, allowing the others to gain the upper-hand against him. Now powerless but still regenerating, Apocalypse is delighted to see the Phoenix, and states that "all is revealed" before the Phoenix transforms him into dust. Jean successfully returns to her normal self, seemingly without any stain of the Dark Phoenix in her.
To be added
- Phoenix Force - As Phoenix, Jean's powers are brought to their full potential, and her psychic abilities are increased beyond their normal limits.
- Reality Warping - The Phoenix has the ability to change reality at will. With her powers contained, Jean is not able to use this power, but when the Dark Phoenix is at full control, she can alter and manipulate reality at will, she can use this ability to manipulate the existence and the time, as well as the life force. When enraged, all matter around her is destroyed if it is too close to her.
- Enhanced Telekinesis - As the Dark Phoenix, Jean has a limitless telekinetic potential. She held a tsunami behind with this power. When scared, confused or angry, her telekinesis could destroy things around her in a random pattern. She was able to hold her house and everything inside it several feet above ground and levitate herself with it, and during her rampage on Alcatraz, her telekinesis started to destroy everything around her and make anything on the island float.
- Enhanced Telepathy - While she was in a cocoon of telekinetic energy, her thoughts could fly miles away into the others' minds. She was able to defend her mind from Professor X's telepathy, even though is the world's most powerful telepath. During the battle against Apocalypse, he, who has shown to be stronger than Charles inside the Astral Plane, was no match for the Phoenix, who easily destroyed him and the Astral Plane altogether with a mere scream.
- Precognition - In both X-Men: Apocalypse and X2: X-Men United, Jean has been shown to be able to see and feel glimpses of future events. Both times, she used this ability it when the Phoenix was trying to break free.
- Energy Manipulation - Jean can manipulate and absorb any kind of energy on a particular level. She can redirect energy and control it, for example, when she was able to stop Scott's eyes from releasing a optical beam of energy, and was also able to redirect it during a fight. To save herself from death, she enveloped herself in a cocoon of telekinetic energy.
- Cosmic Fire Generation - Jean can project a different kind of energy that resembles fire, but is stronger and able to destroy matter, normally as heat and fire around her as a form of a giant bird that protects her. She becomes so powerful that she was able to harm a "Mutant Deity" like Apocalypse, rip off his mystical armor and disintegrate him once and for all.
- Disintegration - As the Dark Phoenix, Jean can destroy her opponents by disintegrating them. During her first disintegration attempt, the Phoenix slowly destroyed Professor X's cells before she completely destroyed him in the apex of her wrath. Later, when she is on the rampage, she disintegrates everything around her with no effort at all. In the revised timeline, during the battle against Apocalypse, she also slowly disintegrated him.
- Enhanced Biokinesis - The Dark Phoenix can also alter the formation of living creatures and cells. She has not only shown to be able to disintegrate organic matter, but also she was able to deactivate Scott's mutation in order to see his eyes and killed him while kissing him by destroying his cells.
- Healing - After the battle against Apocalypse was over, Jean was able to heal Charles' mind from the damage in his mind, done to him by Apocalypse during their struggle in the Astral Plane and the failed transference ritual.
- Jean's use of Cerebro in X-Men was what caused the psychic barriers in her mind to disintegrate, which is why her powers grew stronger in X2 and made her Phoenix persona emerge in X-Men: The Last Stand .
- During X2: X-Men United and X-Men: Apocalypse, the power of the Phoenix Force being used made Jean's eyes flash in a blazing orange color, like they were actual flames. However, during X-Men: The Last Stand, whenever the Dark Phoenix used it's powers, her eyes become fully black with an animal trait to them and dark veins spread through her face, with her hair having a fiery effect.
- This can be a nod to the different use of power between Jean Grey and the Dark Phoenix.