I don't know if this counts, but when I ran Ravenloft in 3E, the party arrived in Barovia and got the usual tales of "The Devil Strahd". Making their way to the castle, they encountered Strahd, who was a peaceable, likeable and pityful fellow. He informed the group the castle was under a curse by an evil spirit that was committing horrid acts across the countryside in his family's name. He was the last of his line, and should he die, the evil spirit would be free to wreck havoc across the land beyond the bounds of Barovia. The party offered to track down the spirit (which lurked in the castle's catacombs) and destroy it, which would lift the curse on his lineage.
As the party made their way through the castle to arm up with gear provided by Strahd, the party began to realize that one of their members had been possessed and was under the control of the evil spirit, who was secretly observing them and waiting for the right moment to strike. Conversely and secretly, the party (via passed notes) made plans to get the possessed character to Strahd's lab where they could overcome the party member and excise the evil spirit.
The plan almost worked - the "possessed" character, who happened to be the group's cleric was isolated and the party used one of Strahd's gifted items to drive the spirit out ... only for the party to realized they'd all been
charmed by Strahd (except the cleric) and he'd used his influence to trick them to turn on the character most likely to be a problem. Unfortunately, they didn't realize what was really going on until
after the cleric was dead and they had returned to the village thinking they'd freed Strahd of his "curse".
Never played a group like a fiddle so well before or after that.