How to get Darkrai in Pokemon Diamond:

To Get Darkrai In Pokemon Diamond Version, You Need to Follow These Steps.

1. Get a Acton Replay With the Membership Pass Card Cheat
2. Start the game and Press L+R in front of the Pokemart.
3. Talk to the green guy and get the card.
4. Go into the Inn and the guy says good night or something, so you fall asleep instantly.
5. You wake up on a island and go into the forest to fight DarkRai-Hes at level 40 in the wild like latias/latios.
6. Throw a UltraBall,Pokeball,GreatBall, Masterball or Whatever you usually use to catch a legendary level 40.
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Another Way:

1. Put on the Action Replay DS on and Select Walk/Run Through Walls Cheat if its there.
2. Start the game.
3. Go To Canalave and Talk to the guy that gives you a ride to Full Moon Island and iron Island.
4. Go to Full Moon Island and Walk Directly -->
5. Then, it will say that you are at newmoon and go into the forest.
6. Darkrai will be hovering above a big puddle of water.

Information:
http://www.pokemon.com/darkrai/
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http://bulbapedia.bulbagarden.net/wiki/Darkrai_(Pok%C3%A9mon)
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http://www.serebii.net/pokedex-dp/491.shtml