Darkrai was a very popular legendary Pokemon in the 4th generation. It could have been found in Pokemon Diamond, Pearl, and Platinum. You would have to get a Key Card through Mystery Gift (or AR) then go to the locked house and keep playing until you get to Newmoon Island, and you would have the chance to fight AND catch Darkrai. This event was in 2009 and DPP are really old so don't get your hopes up in getting him.
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Transfer it from your bottom game or get it from a Pokemon event. Pokemon events are under Wifi Conection but they don't happen often.
Darkrai is not a legendary Pokemon check in the Pokemon Ultimate Handbook and go on Darkrai and it doesn't say legendary under it's name.
You can't unless you hack it into your game 'cause there's no other way to get Darkrai without its special item or without Nintendo giving out an event Darkrai.
Yes, Darkrai is a Legendary Pokémon.
Unfortunately, the event Darkrai was only released in Japan, where lucky people with preordered tickets to M17 could insert a serial code into their game to get the Darkrai. Hope this helps.