Days of Future Past
"Days of Future Past" is a popular storyline in the Marvel Comics comic book The Uncanny X-Men issues #141 and #142, published in 1981. It deals with a dystopian alternative future in which mutants are incarcerated in internment camps. An adult Kate Pryde transfers her mind into her younger self, the present-day Kitty Pryde, who brings the X-Men to prevent a fatal moment in history which triggers anti-mutant hysteria.
The storyline was very popular at the time and was produced during the franchise's rapid rise to popularity due to the writer/artist team of Chris Claremont, John Byrne and Terry Austin. As a result of the storyline's popularity, the dark future seen in the story has been revisited numerous times. The first issue of this storyline was voted the 25th greatest Marvel Comic of all time by the fans in 2001.
This reality in which the story occurs is designated Earth-811 in the Marvel Multiverse.
The storyline alternates between present day, in which the X-Men fight Mystique's Brotherhood of Evil Mutants, and a future timeline caused by the X-Men's failure to prevent the Brotherhood from assassinating Senator Robert Kelly. In this future universe, Sentinels rule the United States, and mutants live in internment camps. The present-day X-Men are forewarned of the possible future by a future version of their teammate Kitty Pryde, whose mind traveled back in time and possessed her younger self to warn the X-Men. She succeeds in her mission and returns to the future, but despite her success, the future timeline still exists as an alternative timeline rather than as the actual future. (The Official Handbook to the Marvel Universe: Alternate Universes 2005 gave the numerical designation of "Days of Future Past" Earth as Earth-811).
Rachel Summers, who was a key player in the original storyline, traveled through time from this alternative future, Earth-811, to the present day and joined the X-Men. Nimrod, the "ultimate Sentinel," followed her to the present and became a foe of the X-Men and the Hellfire Club. Another supervillain, Ahab, later followed her to the present in the Days of Future Present crossover.