Cinderella Castle Mystery Tour was located in Fantasyland, indside Cinderella Castle. It closed in 2006 after a 20-year run. It was an attraction unique to Tokyo Disneyland; it was unique in several other ways as well.
Unlike most theme park "rides," Cinderella Castle Mystery Tour was a walk-through attraction, with a tour guide as your escort. Both the mood and the lighting were dark, with villains from several prominent (and one not so prominent) Disney movies taking center stage. The tour guides were famous for their animated dialog, and helped keep the mood spooky without ever completely abandoning the lightheartedness you expect at a Disney theme park.
After a group of about 30 guests assembled in the entry hall, you embarked on a walking tour which wound through halls and caverns inside and underneath Cinderella Castle. Stops along the tour depicted scenes featuring famous villains from Disney movies. A recurring character (and sort of a secondary tour guide) was the Magic Mirror from the movie Snow White, appearing in the entry hall and at several points throughout the attraction.
For the grand finale, your group joined a second tour group in a large, dark hall. The hall recreated a scene from the all-but-forgotten Disney movie The Black Cauldron, and did so in dramatic fashion. One lucky member of the audience was chosen, taken to a special platform and given a "magic" sword. As the monster in the center of the room reared its evil head, the audience member pointed the sword in the direction of the monster, a light shot out from the point of the sword, and the monster was destroyed. At the end of the tour the lucky audience member was presented with a medallion as a souvenir remembrance of their adventure.
Postscript: The area formerly occupied by Cinderella Castle Mystery Tour is now the Cinderella's Fairy Tale Hall attraction, which opened in 2011.