After reaching the end of the Pokémon Brilliant Diamond and Shining Pearl game, you want to work towards unlocking the National PokéDex. You can only do this after identifying all of the Pokémon in the Sinnoh region, which also means finding the legendary Pokémon in the different games. If you’re playing the Pokémon Shining Pearl version, you need to find Dialga. This guide covers how to get Dialga in your PokéDex for Pokémon Shining Pearl.
You won’t find Dialga in the wild. Instead, you need to speak with an NPC inside Celestic Town. When you arrive at the city, you want to find the building overlooking the ruins, on the second layer of the location. Head inside, and speak to the elder on the left side.
While speaking with them, they show you an image of one of the ancient Pokémon from an old book. From there, you’ll see a photo of Dialga, and it immediately registers the information for them in your PokéDex.
You still need to encounter the rest of the Pokémon in the Sinnoh region, but this might be one of the more difficult ones because of how obscure it is. Later, you’ll have the chance to encounter Dialga for yourself and catch it to add it to your Pokémon collection.
Alternatively, you can always reach out to another player who owns Pokémon Brilliant Diamond to trade you their Dialga to register in your PokéDex.