Pokémon Diamond and Pearl/Hall of Origin

The Hall of Origin is actually not an area of its own. It is contained in the Spear Pillar area. Currently, the only way to get it is to cheat for an event item (note: simply cheating to get the Key Item in your Inventory doesn't have any effect; you must cheat to receive it from the man in the Pokémart). Once you have done this, go to the Spear Pillar. Walk straight until you step on a white tile. It will ask you if you want to play the Azure Flute (the Event item). say yes, and you will play a short tune on it. Upon doing so, a staircase will appear in front of you leading into, well, the air. Walk up it, but make sure to save your game before you step off the staircase. Once you step onto the glass-like area at the end of the staircase, you will automatically challenge Arceus.

Arceus
Arceus is at level 100, the highest level any wild Pokémon has ever been to date. He has amazing stats (also the highest base stats out of any Pokémon to date, at 120 each). He knows the moves Refresh, Recover, Hyper Beam, and Future Sight. Recover and Refresh make him very hard to catch, because he can Refresh away any status conditions and Recover any damage. You're going to have to get lucky to catch him.