When you reach the end of the game and defeat the Elite Four in Pokémon Brilliant Diamond and Brilliant Pearl, the next step is to unlock the National PokéDex. You can only do this if you encounter all the original Pokémon from the Sinnoh region. They are scattered throughout the game, many of them appearing in manager battles, and some of them are in the wild on specific courses. However, the Legendary Pokémon counts well, which means you need to find Palkia for the Pokémon Brilliant Diamond version. In this guide, we explain how to get Palkia for your PokéDex in Pokémon Brilliant Diamond.
You have to reach Celestic Town. You can find this town near the center of the map, along Route 210 and to the right of Mount Coronet. When you arrive, head to the north part of the city and look for the large building overlooking the ruins on the third layer of the city.
Inside the house, talk to the elderly woman on the left side of the room. She will tell you about reading old books and, eventually, she will show you a picture of Palkia. When you record this information in your PokéDex, make sure you are one step closer to unlocking the national PokéDex.