Halloween One-Shot

A few years ago I decided to run a special Halloween D&D one-shot adventure.  Not Halloween-themed, but dark and/or creepy rather than our traditional stuff.  No planning went into this, the entire thing was improvised, and what resulted was not a one-shot adventure but rather an epic, sprawling time-travel campaign that lasted about 25 sessions and almost 2 years. 

We made up some half-assed reason the party was together in town, and didn't spend too much time on it.  There was a festival in town, and the players happily took part in a number of contest and events, and basically just had some fun roleplaying and beating NPCs at trivial tasks. The main event was a magician, who called them, the "champions of the festival" up on stage as guests of honor. One of his main tricks involved copious amounts of brilliant fireworks, and also flying around above the audience tossing pyrotechnics into the air.

The PCs slowly realized that the fireworks and explosions were scattering strange black sand/beads into the audience, and anyone touched by said beads started acting really messed up, freaking out, hallucinating, etc. They started investigating, and about an hour into their investigations I dropped the kicker on them - an NPC who had just arrived in town and hadn't been affected by any of the sand asked them why there was sand in their hair. They then realized everything they had been seeing and reacting to was all not as it seemed, and from there things went downhill. It ended with the entire town turning into shadow creatures and the party leaping into a rippling black liquid filling the central town fountain, and emerging into a pristine cavern with 12 perfect white doors around the room like a clock face.

Over the course of the campaign, the players went through the (magical) doors to 12 different locations. After a while they deduced from talking to NPCs that not only were they traveling to different places, they were also traveling to different points in time. In each location, they were tasked to find a crystal and destroy it, which would bring them back to the room with the doors, and the door they had entered would crumble to dust. They went to the arctic, a haunted forest, an orc stronghold in the mountains, a desert excavation site, a city under siege from an army of demons...each door was a totally new setting, with new scenarios playing out.  Each of the locations also had a subtle theme, such as war, madness, necromancy, etc.

Had it not ended early, the campaign would have seen an international uprising of non-human races rise up against their oppressive human overlords (in four different countries simultaneously), and a resulting backlash against the rebels by human-piloted Warforged Juggernauts (think X-Men's Sentinels, both in flavour and function).

The rebellion itself was also part of a larger plan to destroy the human nations' capital cities, which would shatter a long-forgotten magic barrier shackling a floating island stronghold to the bottom of the sea. The owner of the stronghold, the recently freed Vecna (who had been trapped in Ravenloft until the seals trapping him there - the crystals - were broken), would then be free to raise his fortress and seek revenge.

This was all part of an even larger plan, as the mastermind behind it all, Vecna's former lieutenant Kas (actually an ex-PC after a player decided to change characters), would finally be able to seek his centuries-old revenge on his now-mortal ex-boss.

Not bad for what was supposed to be a little improv session.