In this Ravenloft adventure, PCs suddenly find that they are now inhabiting the bodies of unknown characters at a ball held, as far as I remember, by Strahd von Zarovich at his castle.
As the PCs are trying to go with the flow, most probably trying to escape the ball and the castle, some world-shattering event happens and everyone at the castle dies, with the PCs waking up as heads-in-a-jar on the shelf of some powerful Ravenloft NPC.
The NPC explains them that he has been able to transfer their conscience to the body of participants to the ball (henceforth PttB), which happened in the past. During the ball, something terrible happened and the NPC wants to prevent it.
They will be sent to the same instant in the past again, possessing a different set of PttB. The previously possessed PttB will do the exact same things they did while they were possessed by the PCs on the previous trip (so, for example, if PttB1 went and switched a lever in trip 1, it will do the same in trip 2, but PttB6 can run to the lever and jam it, or kill PttB1, if they discover that pulling the lever was a bad idea. Then on a later trip PttB13 can persuade PttB6 to let PttB1 pull the lever and so on.
Between trips to the past, PCs can talk to each other to coordinate a strategy. They only have as many trips as it takes to possess each PttB once.
What is this adventure called?
I remember the adventure being crudely formatted, so I'm not entirely sure it was an official one, but I'm writing anyway hoping that it was.