How to receive the Oak's Letter via Mystery Gift:
- Launch Pokémon Brilliant Diamond or Pokémon Shining Pearl from the Nintendo Switch HOME Menu.
- Press the X button during gameplay to access the in-game menu.
- Select MYSTERY GIFT.
- Select Get via Internet.
- Select Oak's Letter Gift.
- Be sure to save your game after receiving your gift.
The Shaymin will first appear in its Land Forme and is able to change into its Sky Forme using the Gracidea item.