The Pokémon Company is currently giving away the Mega Evolution Stone for Mewtwo, The Mewtwonite X & Y for Pokémon Sun and Pokémon Moon.
You just need to use the code: M2DESCENT on the game’s mystery gift feature found on the main menu.
I know this may be weird especially since you can’t get Mewtwo in-game. Even the update also mentions that the only way for you to get a Mewtwo in Sun and Moon is by getting the Pokemon through the Pokémon Bank. The game’s paid storage service.
Here’s the exact process on how to get your Mewtwonite X & Y:
- Select Mystery Gift on the main menu.
- Select Receive Gift.
- Select Get with Code/Password, then Yes, and then Yes again to connect to the Internet.
- Enter your code.
- Watch as you receive Mega Stones.
- Speak to the deliveryman in any Pokémon Center to pick up your Mega Stones.
- Be sure to save your game!
For more info and updates, you can head to the Pokémon site.