Pokémon Diamond and Pearl/Newmoon Island

This island is home to the Legendary Pokémon Darkrai. Currently only way to get this Pokémon is with a walk through walls cheat with an Action Replay. 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.