Pokémon Diamond and Pearl were the first of the fourth generation of Pokémon games, and the first core-series games on the Nintendo DS. They brought back and standardized old features such as day/night cycles (from Pokémon Gold and Silver), wireless online battles and trades (from Japanese Pokémon Crystal)... Of course, there's also new Pokémon and other new mechanics as well.
An event item exists known as the Azure Flute, which was never officially distributed and can only be obtained through hacking the game. When hacked into your Bag, the Azure Flute can be used at Spear Pillar to warp to another map, the otherwise-unused Hall of Origin above Spear Pillar. Atop a glass staircase, on a platform high in the sky, you encounter a level 80 Arceus. It seems this event was the originally-planned method of Arceus distribution, but because the event was thought to be too confusing for players, Arceus was distributed directly as an event Pokémon instead.
Interestingly, it seems that an Azure Flute distribution hadn't been ruled out as late as the releases of HeartGold and SoulSilver; those games have a mechanism to allow a Hall of Origin-caught Arceus to return the player to the Sinjoh Ruins and get another one of the trio of Dialga/Palkia/Giratina.