From Bulbapedia, the community-driven Pokémon encyclopedia.
Dome entrance (bottom),
Challenge Dome arena (middle),
Legendary Card room (top)
The Pokémon Dome (Japanese: ポケモン ドーム Pokémon Dome) is an arena found exclusively in the Pokémon Trading Card Game and Pokémon Card GB 2. After the player receives eight Medals from the Club Masters and completes specific tasks, they are granted access to the Pokémon Dome. It is home to the Grand Masters, who must be defeated before the player can become champion.
The entrance of the Dome is accessible to all players whether or not they have acquired all eight of the club Medals. It is a small wide room with red carpet, a PC, and six bookcases. Four of the bookcases contain information regarding the Legendary Cards, while the other two contain information about the Grand Masters, and the eight Master Medals.
The main room of the Dome is the arena, which consists of the same red carpet as the previous room, but more notably, a large blue stage with a table in the middle. This is where the player battles all four of the Grand Masters, one after the other. After they have all been defeated, Ronald appears to challenge the player.
Legendary Card room
After the player defeats all of the Grand Masters and Ronald, the player is given access to the room containing the Legendary Cards. The room is moderately sized and contains a floor with a golden finish. In the middle of the room is a device which protects the Legendary Cards. The player gets one of each card the first time they arrive here, but only one chosen at random the next time they defeat the Grand Masters. After receiving the Legendary Cards, the player's deck gets entered into a computer and saved, similar to the Pokémon Versions and their Hall of Fame. The player also gets access to the Legendary Auto Deck Machine.