Arceus is known as "The Original One" as Sinnoh legends say it created the entire universe. Arceus's resting place, the Hall of Origin, can be reached by playing on the Azure Flute. Arceus is genderless, and according to the creature's legend it not only created the universe, but also Dialga, Palkia, Giratina, and the three lake Pokémon Uxie, Mesprit, and Azelf. Arceus is able to change it's elemental type to any by using the different types of plates. In this way, its body changes colors to reflect its new type and its signature move, Judgment, also changes type according to the plate. Despite being the "creator", Arceus cannot learn all moves, which is the main difference it has with Mew. Specifically, it seems it cannot learn punch-based moves. It can, however, use the most amount of different attacks of any Pokémon (excluding Mew, Smeargle, and Ditto which can use any moves from the opponent).
A Pokémon said to have hatched from an egg that appeared from nothing. Newly hatched, it created the world and the sky. Its Gravity attack will bring any fighter in midair crashing down to earth. No ground below you? Looks like you're going to go plummeting off the stage as if you'd been hit by a meteor smash!
Nintendo DS - Pokémon Diamond & Pokémon Pearl - 09/2006
It is told in mythology that this Pokémon was born before the universe even existed.
Arceus has only been able to be obtained legitimately through events. However, in Pokémon Diamond, Pearl and Platinum, if players can hack in the Azure Flute item, they can obtain an Arceus from the Hall of Origin.