The title of a Japanese manga series by the group CLAMP, serialized in the monthly magazine Asuka. Published in the U.S. by Viz Comics under the name X/1999. Also made into a movie and a twenty-five episode anime.
The story centers around the teenage boy Kamui, who must decide the fate of humanity by choosing to fight for its preservation as a Dragon of Heaven or to fight for its destruction as a Dragon of Earth. The Dragons of Heaven and the Dragons of Earth, a colorful mix of eccentric individuals with unique supernatural powers and private motives for fighting, assemble in present-day Tokyo to battle for the Earth.
The authors CLAMP have promised a different ending for all three tellings of the story, feeding various theories about the only one yet to be revealed: the end of the manga, currently on its eighteenth volume. In Japan, each volume features characters from the story as figures in The Major Arcana of a Tarot deck, leading people to guess that the series will run twenty volumes, from The Magician through Judgment. X Zero, featuring The Fool, was an artbook, and it seems likely that X Twenty-One, with The World, will be its partner.
Unfortunately, the X manga is currently on indefinite hiatus, with no new installments since May 2003. CLAMP's focus has been diverted to its two newest series, XXXHolic and Tsubasa RESERVoir CHRoNiCLE, and there is no telling when they might return to X. I have heard people call it their never-ending story, given that it was begun in 1992, and one would assume that CLAMP intended to finish it sometime before or during what in the story is the fateful year of 1999.
What exists of the manga is a somewhat slow-moving tale, full of darkness and angst and pretty boys, interspersed between much destruction and gore. It has an interesting plot, with many twists and turns, and introduces many philosophical points worth pondering. And CLAMP are, as always, mistresses of the unexpected.
As for the movie, it is your perfect example of what happens when you try to pack too much story into too little time. It can be easily summarized:
"Dragon of Heaven, meet Dragon of Earth. Fight. Die. Next."
Its only redeeming factor is gorgeous animation, but that's a hard sell against the lack of character development.
The anime series is a infinite improvement over the movie. It keeps the beautiful art, and adds a more faithful and detailed retelling of the manga, although with some departures, of course, and its own unique ending.