The Mythical Pokémon Darkrai was first available at GameStop stores in May. If you weren't able to make it to GameStop in time, here's another chance to receive the Pokémon. Simply enter this code into your Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X, or Pokémon Y game to add Darkrai to your collection:
Code: Darkrai20
It's the same Pokémon that was given out at GameStop, so if you received that one, you won't be able to get another one.
Don't miss this second chance to get the Mythical Pokémon Darkrai. This code is valid until October 31, 2016, so be sure to enter it by then. And be sure to visit Pokemon.com/20 for more details on the Pokémon 20th celebration.
Source. As someone who decided to complete the Pokédex after all these years, this is great news!
The Darkrai is level 100 and comes with the moves Dark Void, Nightmare, Ominous Wind and Faint Attack, plus its signature Ability Bad Dreams.
UPDATE: European gaffers can get Darkrai too, but they have until the 30th of this month to do so, so be quick!
Update: The distribution has now been officially announced, confirming that it will end on September 30, 2016.
Darkrai
A second chance Darkrai is now available for PAL region Pokémon X, Y, Omega Ruby, and Alpha Sapphire using the common serial code DARKRAI2016.
It cannot be obtained on the same game as the Darkrai from the earlier Pokémon 20th Anniversary distribution, and is identical to it.