Pokemon Diamond Unlockables

Beat the elite four at night time, then walk over the water to full moon island.

After you beat the Elite Four, go to the Snowpoint Temple, travel it, and at the bottom you will see a statue of the legendary pokemon Regigigas. You will have to obtain: Regirock, Regice, and Registeel to make it appear. You will have to obtain the three through trade or migration=bringing them from a Game Boy/Advance game(Game Boy Color games will not fit in the slot). Either way, you can not catch the three from diamond(without action replay).