Mystery Gift has been a feature in Pokemon that allows players to connect their Nintendo system online to receive a gift from The Pokemon Company. Whenever there is a special event, players can use Mystery Gift and the Pokemon will show up in the players' game in the Pokemon Center. If Pokemon Brilliant Diamond and Shining Pearl players want to unlock Mystery Gift so that they can receive special gifts, like a Manaphy Egg, from the game developers.
To unlock Mystery Gift in Pokemon Brilliant
Read more on gamerant.com