When you reach the end of the game and defeat the Elite Four in Pokémon Brilliant Diamond and Shining Pearl, the next step is to unlock the National PokéDex. You can only do this if you encounter all of the original Pokémon from the Sinnoh region.
They’re scattered all over the game, many of them appear in trainer battles, and some of them are in the wild at specific routes. However, the legendary Pokémon counts well, which means you need to find Palkia for the Pokémon Brilliant Diamond version.
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