We play Assault on Castle Ravenloft every year.
Here's how it goes...
The Mists decend on your current group's characters, when it lifts they have been transported to Madam Eva's camp near the base of the waterfall. It is already night.
Welcomed into the camp and lead to Madam Eva, the party learns they are expected, it happens every year, a group of heroes arrives to try to destroy Strahd.
Run players through the Taroka reading to determine the locations of everything WITHIN Castle Ravenloft. Madam Eva also reveals that the heroes have always perished.
Inform players that this is a one shot adventure, their only goal is to destroy Strahd and survive. The Taroka reading constitutes the party bring Geased by a major artifact with no save.
Then March of the Dead happens, the ghostly procession to the castle. The party immediately follows the last member who eerily looks back at them. Each character thinks that apparition looks too much like them. Any character resisting gets dragged along as if they were a puppet on strings.
The party gets left at the drawbridge, set timer at this point to be close to when the game will end, in game it will corespond to midnight. If Strahd is still around at that point, GM plays him as a genius level PC intend on hunting down and destroying the party as quickly as possible using every creature within the castle to the fullest of their abilities.
Again the goal here is to destroy him as quickly as possible, allow the party to do whatever the want within the castle or throw themselves from the battlements if they wish. Let them grab whatever loot the want, it all doesn't matter.
To spice things up, add whatever favorite former PCs from your games to the castle. Just put them wherever feels best, as a dead body. In our game, ALL the previous dead PCs are still where they fell, even if it is the same character!
When a PC dies, their vision fades to black, they will regain consiousness the next day where the Mists took them. Any death wounds will now be an old faded scar.
Surviving PCs only gain some minor reward like a potion or something, or you could reward them based on how well the died.
The purpose of this is to play the best adventure ever, quickly!
This is a one shot game, that is only played in one sitting.
Any consumables used by the party are replaced after the game is over. Any loot aquired turns into worthless copies.
All returns to as it was before except for scars, a minor item still in in their possession, and their memories.
Players will also get to experience a Boss Monster unleashed, played the way it should with no restrictions!
It's as close as playing in a Dracula movie as it gets! Players will scream as they die or throw themselves from the battlements to avoid death by undead!
Use plenty of mood setting ideas, the black velvet tablecloth, lower lighting, mood music etc..
Make sure you are familiar with all the special rules for Ravenloft and use them. Adjust whatever you need to for more powerful groups, do not adjust anything to allow the players to succeed if they cannot.
Then next year when the last apparation turns to look at them they see.....