Dark Phoenix

Dark Phoenix is the name given to Jean Grey after she became corrupted by the Hellfire Club and turned into a vastly powerful supervillain who proceeded to cause chaos across the universe until being defeated in a battle than supposedly claimed her life (of course Jean Grey was later revealed alive) - although modern retelling attribute much of the events to the corruption of the Phoenix-Force in the original comic book (and X-Men 3) the Dark-Phoenix was simply Jean Grey gone bad rather than a separate/merged entity.

The Phoenix, in the form of Jean, appeared as the primary antagonist of X-Men: The Last Stand and a prominent plot point in the final episodes of Wolverine and the X-Men. Dark Phoenix was also used in the 1990's X-Men cartoon's third season. She was portrayed by actress Famke Jansen in X-Men: The Last Stand.

X-Men: The Last Stand
Dark Phoenix is the secondary, later true main antagonist in X-Men: The Last Stand - this time caused by Jean apparently going insane. In the film, Professor Xavier recognizably remembered this alternate personality of Jean Grey by its other name "The Phoenix", whom he sees as in his own words, "a creature all desire, joy and rage".

X-Men: The Animated Series
Dark Phoenix also appeared in the animated series X-Men, in its third season as the true main antagonist. In this series, the ending of the Phoenix story arc shows Jean Grey, as Phoenix (still as a fiery creature), taking the super-powerful M'Kraan Crystal into the core of the Earth's yellow sun. Oddly, both survived this and were eventually brought to Muir Island, where Phoenix was developing a darker side that Xavier was understandably scared of. Phoenix would refuse to abandon Jean's body, which leads to her getting captured by the Inner Circle (Hellfire Club) and mind-controlled. Dark Phoenix emerged and broke free of the Inner Circle's control, going into space to destroy a star that was monitored by Shi'Ar explorers. Shi'Ar queen Lilandra goes to Earth and demands that the Dark Phoenix be executed due to it being able to destroy "all that is", just like the M'Kraan Crystal that spawned it. The X-Men have to battle the Shi'Ar's gladiators to save Jean, but when her powers become dangerous, she uses a Shi'Ar space cannon to kill herself. Phoenix emerges and, with the X-Men's life-force, resurrects Jean and leaves.

Wolverine and the X-Men
The Phoenix is featured as the hidden tertiary antagonist in the TV series Wolverine and the X-Men. In this series, the Phoenix is a creature that has been around on Earth since homo superior first appeared. It would enter the host mutant at birth, grow within it and then reach maturity, at which point it would cause mass destruction and risk total destruction of Earth. At the start of the series, a mysterious explosion occurred at the X-Mansion that presumably killed Jean Grey and Professor X.

A year later, Magneto finds a comatose Professor X on the shores of his kingdom of Genosha. The Professor is brought back to the X-Mansion, where the Professor reveals that the coma he is in will last for 20 years and he has woken up to see a world ravaged by fire and the robotic Sentinels are making mutants an endangered species. The cause of both the explosion and the fires on Earth were revealed towards the end of the series, when Emma Frost and Cyclops look at the X-Mansion before it is destroyed; both were caused by stress from Jean Grey, the unfortunate host of the Phoenix, who would unintentionally unleash her powers.

In "Shades of Grey", Jean Grey (who has recently woken up from a coma in a hospital) is ratted out to the Mutant Response Division, who comes to capture her. The Phoenix suddenly manifests and unleashes a psychic wave that causes a portion of the East Coast to lose consciousness. Cyclops and Emma save Jean from Mister Sinister, who uses her Phoenix powers to defeat him. Emma brings Jean before the Inner Circle and tells Jean of the Phoenix, what it can do and how they can stop it; by getting the Stepford Cuckoos to absorb the Phoenix. However, when Emma discovers what the Inner Circle plans to do with the Phoenix, she absorbs the fiery creature into her body, turns into diamond and explodes, releasing the Phoenix into the atmosphere and killing both of them.

Marvel: Ultimate Alliance
In the first game, Dark Phoenix appears at the end of the game. If the player decided to save Nightcrawler instead of Jean Grey in Mephisto's realm, Dark Phoenix will come to Earth and take revenge on the people who refused to save her.

In Marvel: Ultimate Alliance 2, Jean Grey is playable with the Phoenix and Dark Phoenix outfits being available to her.

Dark Phoenix appears as a playable character in Marvel vs. Capcom 3: Fate of Two Worlds. If Phoenix's life gets empty and her Hyper Combo Bar is full, she will turn into Dark Phoenix.

Trivia

 * In X-Men: The Last Stand, Dark Phoenix was the true main antagonist because she had bigger plans and was more powerful than Magneto.