As we all know, the X-men have had numerous timeline stories. Perhaps the most famous is the Days of Future Past saga, with many new characters coming up out of this future into our present and remaining stedfast in their past.
Professor X's dream has become a nightmare. Mutants are either imprisoned or slaughtered. Sometimes both. All of this began when Mystique and her incarnation of the Brotherhood of Evil Mutants assassinated Senator Robert Kelly. This was done in a rather backward-thinking attempt to warn normal Americans not to take legal action against mutantkind. This assassination inflamed bigotry, hatred, and fear among the populace and the feds dispatched a legion of Sentinels to dispose of the mutant problem once and for all.
The Sentinels, being sentient beings, decided that the best way to protect the human race was to take control of it.
Thus, many of the X-Men were killed or captured in concentration camps. The surviving X-Men regrouped with Magneto as their leader, now as crippled as Xavier was. The other survivors were Wolverine, Colossus, Franklin Richards(son of the Fantastic Four's leader), and Rachel Summers. They devised a plan to save their past lives from this present hell. A young mutant named Rachel Summers (yes, Jean and Scott's daughter!) transported a middle-aged Kitty Pryde, now called Kate, back in time into her younger body. Wolverine then busted them out of the camp and they lay in hiding with Kate's prone body for as long as possible from the Sentinels. Kate was then to warn the others of the imminent danger ahead should Kelly be killed.
The future X-Men broke into the Sentinels core, but Wolverine waskilled with one blast from a Sentinel's hand, leaving his adamantium skeleton behind. Storm and Colossus were also killed and Rachel was the only one left to protect Kate's body.
Senator Kelly's life was saved and Kate was instantly transported back to her own timeline. Strangely enough, her self travelled back to the Days of Future Past future, while the X-Men of the past followed a different route. Thanks to Kate Pryde's sacrifice, the X-Men saved themselves and countless others from a fate worse than hell, but remain with the knowledge that they must guard against what may be.