This is a wonderful question! Sure, you could adapt an existing horror adventure but it could easily feel like you did just that. If you want it to be special, it really needs to be custom from the ground up (imo). Other than lighting and music I don't use props, but for a holiday horror I would suggest working in music from "A Very Scary Solstice"
Play it softly, so you almost can't make out the words. It will sound like you have carols playing and it is the holiday season. As already mentioned, part of horror is taking the familiar and twisting it. Over the course of the evening you can up the volume a bit at a time. When they do finally make it out it will creep them out. If you need to hear one this is a fav of mine, Carol of the Old Ones.
As for the story and it's elements that is a matter of taste. Let me tell you what I would do. Please keep in mind this is all off the top of my head.
There has been a good bit of focus on Santa and elves so let me take it in a different direction, the yule log... Yule log, it's big, it's heavy, it's evil wood!
A remote village, blanketed in snow. The air is crisp and cool. The local woodcutter, Olaf, and his two eldest are off to harvest a tree for this year's holiday season. It's for the whole village so it needs to be a big one and Olaf has just the tree in mind. He spied its monster branches rising from the mist over a week ago and just that glimpse was enough for his well trained eye to gauge it a prime specimen. That and the fact that harvesting one huge tree would take less time than five smaller ones. Which means more time for beer! Which was what Olaf did with most of this past week rather than scouting for more trees.
Olaf will go and cut down the strange and huge tree. But that is just the beginning. This tree is fundamentally wrong, bad, evil, jinxed, maliferous, the last slice of pie. When burned, the wood will release its magical toxin which alters the fabric of reality, bringing nightmare versions of the idea of joy found in the minds of nearby sentient beings. Or just go for their deepest fears if you like.
What happens? Anything, everything, whatever you want! As more wood burns the effects get stronger. Here are some examples off the top of my head:
- A parade of tiny, walking, sweet cakes that are intent on force feeding themselves to a victim until they die. (name that movie?)
- Gifts with wrapping that strangle you.
- Summoning evil spirits to eat the children.
- Turns all the bad children into savage little gnome like beasts willing to kill to feed their sweet tooth.
- Mulled wine that sends the drinker into a murderous frenzy.
- Raise the dead as a horde of flesh eating snowmen.
- Opens the portal for an elder god.
You do have to ask why the wood is like this. Well, you don't, but someone will. It could be an insane tree spirit. Or the tree was a prison for a demon. Or a hiding place for an nightmare entity that has just escaped the world of dreams.
How would it play out? Like a mystery? Say the days before the holiday there are a couple of deaths, bloody awful affairs. It's just some stray bits of the evil tree being burned in the victims' proximity. Or you can go the route of full blown Armageddon! No warnings, every house in the village burns the logs and all hell breaks loose. Or the path of aftermath. It happened a day or two ago. The evil stuff is still about and the town is found in a state of post-apocalyptic-ness, maybe a few survivors scrap for survival and/or escape.
