Arceus was first introduced in Pokemon Diamond and Pearl's sequels, Pokemon Platinum. In Platinum, you can encounter Arceus on a special island called "The Azure Bay" after you have completed the main story and obtained the National Pokedex.
However, you can still obtain Arceus in Pokemon Diamond through various methods like:
* Trading: You can trade for Arceus from players who own Pokemon Platinum or other games where it is available.
* Events: Nintendo occasionally hosts events where you can receive Arceus. You can check their official website for updates.
* Hacking: This is not recommended, as it goes against the game's rules and can lead to bans.
While Arceus may not be directly encountered in Pokemon Diamond, it is a very popular Pokemon and is definitely worth seeking out through other means.