It seems like you might be thinking of a Rare Candy, which is a very valuable item that allows a Pokémon to level up instantly.
Here's how to get Rare Candies in Pokémon Diamond:
* Trade: You can trade for Rare Candies with other players.
* Cheats: If you're using an emulator, you can use cheat codes to get Rare Candies.
* Events: Rare Candies may sometimes be given out as part of special events.
It's important to note that using cheats or exploiting glitches to get Rare Candies can be considered cheating and might make the game less enjoyable.
Remember, the best way to level up your Pokémon in Pokémon Diamond is through battling and training.