The Reveal Glass was first seen in Meloetta and the Undersea Temple!, where Team Rocket used Meloetta's recorded song in order to open its seal inside the Abyssal Ruins. Giovanni then used it to summon and subsequently transform the Forces of Nature into their Therian Formes, so he could use them to take over Unova. When Ash's Pikachu freed Meloetta with Electro Ball, Giovanni was overcome by the Reveal Glass's power, causing him to send the Legendary Pokémon on a rampage to destroy the region. The possession was broken when Jessie, James, and Meowth pushed Giovanni out of the spell circle. After Team Rocket's subsequent retreat, Ridley used the Reveal Glass and Meloetta's song to calm down the rampaging Legendaries, after which both the Abyssal Ruins and the Reveal Glass sunk beneath the waves once again.
In the manga
Reveal Glass in Pokémon Adventures
In the Pokémon Adventures manga
The Reveal Glass made its first appearance in PS537, where it was revealed to be in Colress's possession, using it to transform Thundurus and Landorus into their Therian Formes to fight Blake.
Trivia
Origin
The Reveal Glass may be based on the Demon Detecting Mirror, an item in Chinese mythology that could reflect the true identity of a yaoguai.