Pokémon Diamond and Pearl were the first mainline Pokémon games to feature online play, and that functionality returns once again in the games’ remakes, Brilliant Diamond and Shining Pearl. Online play works largely the same as it did over a decade ago; you have to visit the Union Room, where you’ll be able to battle and trade with other players. Here’s how to enter the Union Room in Pokémon Brilliant Diamond and Shining Pearl.
To enter the Union Room, first head to any Pokémon Center, and you should see two sets of stairs. One leads up, the other leads down one floor. The former will take you to the Local Union Room, where you can interact with players around your physical location who have also entered the room. After defeating Roark, the first gym leader, you can head downstairs towards the Global Union Room, where you’ll be able to play with folks around the world via the internet.
After entering one of the Union Rooms, you can hit the “Y” button while wandering around the game’s map to enter either Union Room at any time, regardless of your present location.
Each Union Room also offers ways for you to filter who it is you’re playing with. If you want to play exclusively with friends, you can set a link code to ensure that the only people who can join your room are those who know the code. Additionally, you can also make a Union Room exclusively for your group, which is comparable to a guild/clan in other multiplayer games.