He'll give you the Explorer Kit, which can be found in the Key Items pocket of your bag, and a short tutorial on how the Grand Underground works. The Grand Underground is unlockable the moment you set foot in Eterna City - simply visit the house next to the Pokémon Centre and talk to the old man inside. This is the third major city you'll visit in the game, containing the second Gym, and takes roughly five hours to reach. To unlock the Grand Underground in Pokémon Brilliant Diamond and Shining Pearl, you must first reach Eterna City.
Watch on YouTube Pokémon Brilliant Diamond and Pokémon Shining Pearl - Overview Trailer - Nintendo Switch How to unlock Grand Underground in Pokémon Brilliant Diamond and Shining Pearl Multiplayer for the Grand Underground of Pokémon Brilliant Diamond and Shining Pearl.