If held by a Mewtwo (or is present in the player's Bag)PE, allows it to Mega Evolve into Mega Mewtwo X if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Mewtwo a Mewtwonite X or remove a Mewtwonite X from Mewtwo.
Outside of battle
If held by a Mewtwo, allows it to Mega Evolve into Mega Mewtwo X when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Mewtwo will remain a Mega Mewtwo X until the Talent Round ends.
The player can obtain the Mewtwonite X by defeating Mega Mewtwo in Stage 350 found on Desert Umbra.
The Mewtwonite X was also available as a prize during the Mega Mewtwo X Competitive Stage. This Competitive Stage ran on both the Nintendo 3DS and Mobile versions from January 3 to 10, 2017 and August 29 to September 5, 2017.
The Mewtwonite X enables the following Mega Evolutions:
A Mewtwonite X and a Mewtwonite Y appeared in The Discovery, under Green's possession. After discovering that Elaine had managed to catch Mewtwo before her and being defeated in battle by her, Green gave the Mega Stones to her.
In PS591, Blaine was revealed to be in possession of both a Mewtwonite X and a Mewtwonite Y, which he can use to Mega Evolve Mewtwo. After Mewtwo's Mega Evolution into Mega Mewtwo Y was undone while battling Lysandre and Zygarde, Blaine swapped its held Mega Stone, allowing Mewtwo to Mega Evolve into Mega Mewtwo X and keep on fighting. Blaine then explained that due to the unique bond he and Mewtwo share, Mewtwo is able to Mega Evolve more than once per battle.
