The legendary Pokémon Dialga and Palkia are locked exclusively to either version of the game, but to get a full Pokédex, you’ll need to register both. Well, how can you do that? It’s simpler than you may think.
To get the first legendary Pokémon in the game, you’ll need to face off against Team Galactic on the top of Mount Coronet. At the climax of the encounter, you’ll get a chance to catch either Dialga or Palkia, depending on the version of the game you have.
After that, head over to Celestic Town. It’s the area you needed to get to after clearing the Psyduck blockade and defogging the route toward it. If you’ve forgotten where it is, Celestic Town is east of Eterna City and northwest of Solaceon Town.
When you arrive, head to the biggest building in this quiet neighborhood. It’s north of the ancient cave. Once you’re inside, go talk to the elder in purple. She’ll show you an ancient book that has information on Dialga or Palkia. Even though you haven’t seen or caught this Pokémon in battle, this interaction with the elder will count on the Pokédex.
Another elusive Pokémon to find in Brilliant Diamond and Shining Pearl is Burmy. It can be found on honey trees throughout Sinnoh.
