Pokémon Diamond and Pearl/Newmoon Island

This island is home to the Legendary Pokémon Darkrai. Currently only ways to get this Pokémon are to walk through walls with an Action Replay cheat or start the event with receiving the 'Member Card' from the man wearing green in the PokéMart. To do so, walk to the right from Fullmoon Island and eventually you will reach the Island.

Go through the entrance to the forest clearing. You will see Darkrai hovering there. Walk up to him and challenge it.