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.