Now available is a free Diancie via the Mystery Gift feature in Pokemon Omega Ruby & Alpha Sapphire. So if you didn't make it to the event at EB Games to collect a "Diancie code," now's a great chance.
To obtain Diancie, load up your copy of Pokemon Omega Ruby or Alpha Sapphire > Mystery Gift > Receive Gift > Get via Internet.
With the gift received, go to any Pokemon Centre and talk to the girl in blue to the left of the Restore Machine. She will give you your Diancie. If you re-enter the Pokemon Centre with Diancie in your party, an dialogue-based event will occur and you will receive the Diancite, allowing your newly received Diancie to Mega Evolve!
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