I played Ravenloft as a player. I DMed Ravenloft II, House on Gryphon Hill.
I started with an evil entity in my game world that was trying to resurrect Vecna. The players managed to find Vecna's "dead" mummified hand. They were staying in an inn, and in the middle of the night, the hand tried to kill one of the characters. He happened to have a javelin of lighting, and struck the hand with it, burning down the inn in the process.
Of course the locals just thought the inn had been struck by lightning. The hand escaped.
Later on, the same character was in a prison cell with a guy who had torn out his own eye, and put Vecna's eye in the socket. A mostly re-assembled Vecna ripped the door off the cell, then tore the guy's eye out, reclaiming it for his own.
They chased Vecna across the sea, and found an entity (Strahd) that was in collusion with him. After destroying Vecna, they chased the entity through a mirror to another dimension... his home. But the entity abducted a woman he was in love with and took her with him.
In this dimension, Strahd is the disembodied evil half of the Alchemist. Like Captain Kirk, the Alchemist is weak because he is missing part of himself. Strahd possesses the corpse of the Alchemist's long dead uncle, and re-animates the body through the sacrifice of a human (sort of like Dracula reconstitutes from the ashes in a couple of the Hammer films). Strahd is now a physical vampire.
Naturally, the uncle and the Alchemist bear an uncanny resemblance (al la Dark Shadows).
The woman has no memory of the other dimension, and they have to figure out who she is in this dimension so they can bring her back with them.
Strahd uses the Apparatus to exchange the souls of villagers with those of demons. This process actually kills the body, so an entire village of intelligent zombies is being created. The demons liked to taunt the characters.
One of the best scenes was when they went to talk to the village priest... he was sitting in the dark.. rocking in his chair... conversing with them.. when he leaned into the light, they could tell he had been dead for over a week.
After finding the vampire lair in the sewers beneath the city and staking all the vampires, and after killing all the zombies, the adventure ended with Strahd and the Alchemist fighting on the cliff. The players destroyed the Apparatus, and in a lighting storm the two were characters fused back into one... and leapt off the cliff.
The characters were able to find their way back to their own dimension at the end.
I know I changed a lot of it, but at this point i don't remember what was actually there, and what I added.