The Odd Keystone (Japanese: かなめいしKeystone) is a type of item introduced in Generation IV. Odd Keystones act like containers, capable of trapping and binding Spiritomb inside themselves. All Spiritomb use it as a base.
If the player talks to 32 or more other players in the Sinnoh Underground after placing the Odd Keystone, a wild Spiritomb will be encountered the next time the player inspects the Hallowed Tower. While in theory it is necessary to talk to 32 different players, in practice another player can be counted twice as long as the player themselves leaves the Underground or enters their Secret Base using the "close door and decorate" function between the interactions; this potentially allows a player to encounter Spiritomb by interacting with the same player 32 times.
Once placed in Hallowed Tower, the player can check the Odd Keystone to track their progress. The message gradually changes as the player interacts with more players in the Underground.
In The Keystone Pops!, while Ash and Dawn were having a practice battle between Buizel and Aipom, Buizel's Sonic Boom accidentally destroyed the Hallowed Tower, releasing a Spiritomb from the Odd Keystone sealed inside of it. Ash and Pikachu later battled Spiritomb, eventually defeating it and resealing it back into the Odd Keystone, which was then placed back into the rebuilt Hallowed Tower.
In The Spectral Express!, Ash and Goh came across an Odd Keystone aboard a ghost train. When Ash touched it, one of Spiritomb's spirits entered his body and possessed him into doing mischievous deeds. Angered by losing one of its spirits, Spiritomb emerged from the keystone and went on a rampage. Once the spirit was forced out of Ash's body and Ash's Gengar succeeded in returning it to Spiritomb, the Forbidden Pokémon calmed down and returned into its keystone. The keystone was later taken away by a group of Gastly and Haunter when the ghost train stopped at a cemetery.