A Mythical Bonus
Nintendo tweeted a reminder to all players of Pokémon Brilliant Diamond and Shining Pearl of a limited time in-game bonus available now through March 27th.
After opening the letter and completing a short task, you can receive the Mythical Pokémon Shaymin via the Mystery Gift option in the game. For new players, it should be noted that the official Pokémon website says it takes roughly about two hours of game play to unlock the feature. Further, a paid Nintendo Switch Online membership is not required to receive your Shaymin.