"The Dark Phoenix Saga" is an extended X-Men storyline in the fictional Marvel Comics Universe, focusing on Jean Grey and the Phoenix Force, ending in Grey'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 stories in mainstream American superhero comics, and widely considered a classic.
It was adapted for the X-Men animated series, and alluded to in the movie X2: X-Men United. A third movie, X-Men: The Last Stand, released in 2006, contains some elements from the saga. Wolverine and the X-Men adapted the Dark Phoenix Saga at the end of its first season, changing many things to make it a fresh and new version.
In comic books, readers know the Phoenix as a psionic cosmic entity linked to Jean Grey. 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 voluntarily restrained her powers afterward in order to keep them under control.
Read more about Dark Phoenix at Wikipedia ...
Official Site: Marvel
Links: Wikipedia: Dark Phoenix Uncannyxmen.net: Dark Phoenix Marvel.com: The Phoenix Force