How do you get bagon in Pokemon Diamond?

You can't get Bagon in Pokémon Diamond.

Bagon is exclusive to Pokémon Pearl and Platinum. To get Bagon in those games, you need to:

* Find it in the Grand Underground: Bagon can be found in the Grand Underground, specifically in the "Rocky Cave" and "Iron Mine" areas.

* Use the Poké Radar: The Poké Radar is an item that helps you find Pokémon in the Grand Underground. Using it can make finding Bagon easier.

While you can't find Bagon in Diamond, you can always trade with someone who has it!