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:
In Mewtwo Angered, 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.