How do you catch the rarest Pokemon in Diamond?

The rarest Pokemon in Diamond is Darkrai. To catch Darkrai, you must have the Member Card which is only obtainable through a promotional giveaway. Once you have the Member Card, you can go to the Newmoon Island by talking to the sailor in Canalave City. Once on Newmoon Island, go to the ruins and interact with the lunar statue to start the battle with Darkrai. Darkrai is level 50 and knows the moves Shadow Ball, Ice Beam, Dark Void, and Nightmare. It is recommended to use a Pokemon that knows Fighting or Bug type moves to take advantage of Darkrai's weaknesses.