"Demon"
Uncanny X-Men #143, with a cover date of March 1981, is a Christmas Story that mostly focuses on new X-Men member Kitty Pryde. It was written and co-plotted by Chris Claremont, pencilled and co-plotted by John Byrne, and inked by Terry Austin. This was part of Chris Claremont's legendary run on the X-Men, and the fact that top talents like John Byrne and Terry Austin were "just helping" says something about how good this era was.
The story starts with a synopsis of a story from some years previously, with Storm fighting demonic creatures that resemble Xenomorphs. We are then returned to the present day, where we get a little recap of the X-Men and their personalities--- bashful Colossus, frenzied Wolverine, etc. But the focus is on young Kitty Pryde, who is facing the difficult of having to go to school full time...and learn to be an X-Man. Being thirteen and having to learn to fly a supersonic plane is pretty stressful, apparently! Then, Kitty is left alone in the X-Men Mansion on Christmas Eve (although, of course, Kitty is Jewish and mentions "spending Hanukkah alone"), when the Ngarai attacks. Kitty uses her phasing power to run through the mansion, but the supernatural N'garai can still harm her. After destroying the Danger Room, she finally manages to lure the demon in front of the burners of the Blackbird, successfully frying it to a crisp, and saving herself. The X-Men then return, and despite the damage, are proud that Kitty defended herself.
A few things about this story make it succeed: Chris Claremont is a dense plotter, with lots of description of scenes and emotions. Interestingly enough, amongst the many word balloons, thought balloons, and narrative boxes, there is no discussion of what would become the primary theme of the X-Men: the discussion of tolerance between mutants and humans. While having a few other references to X-Men continuity, the story managed to be self-contained. John Byrne's art is also a big asset, in that it allows an aspect of "normality" that make the horror more scary. One of John Byrne's strengths was making "normal" backgrounds, so we really get the feeling of domestic life being interrupted by something out of the normal---even if that "domestic life" is a group of super heroes living in a high-tech compound.
What I liked most though, was that the story actually came to a conclusion, and an emotional conclusion. While it uses continuity, it uses it for the purpose of the story, and not vice-versa. The point of the story is Kitty Pryde being smart and brave enough to defeat a villain, and that is the conclusion. Before long, (and until the present day), the reappearance of the N'Garai would have been a prelude to a gigantic crossover story. But here we have a story that is just that: a story.