As part of the ongoing celebration of Pokémon’s 20th anniversary, there has been a steady flow of legendary Pokémon being distributed every month. Another such event is going on and it has brought back Darkrai, the legendary Dark-type that was previously released in May through GameStop.

Anyone with Pokémon X, Y, Alpha Sapphire, or Omega Ruby will be able to acquire the creature by using the code Darkrai20. Those who grabbed grabbed Darkrai back in May won’t be able to get him again. The code will be valid through October 31.
Source: Pokémon.com