Eager players of Pokémon X & Y, in addition to Omega Ruby and Alpha Sapphire, have been receiving monthly gifts of Mythical Pokémon this year. So far we've had Jirachi, Mew and Celebi, and next up - as you can see in the image above - is Darkrai.
The Pokémon Company has done a mix of online and serial code distributions so far, and the indications are that Darkrai will be shared with the latter method through stores. In the US and Canada GameStop will have dibs on them, while GameMania will have them in the Netherlands, with the same chain being joined by FNAC in Belgium. BR Stores will have them in Sweden and Norway, while GameStop is distributing in Italy; we're waiting for more details on other countries.
As for what Darkrai will have to offer, the following is from the official Pokémon site.
Darkrai's power comes from its high Speed, its Dark Void attack, and its Bad Dreams Ability. Dark Void can put all adjacent Pokémon to sleep in a Double or Triple Battle, a tactic that has certainly haunted some players' dreams. Dark Void is even more of a nightmare when used by Darkrai, as Darkrai's Bad Dreams Ability deals damage to its sleeping foes at the end of each turn. Bad Dreams can effectively negate an opponent's Focus Sash item or Sturdy Ability by delivering a pretty sizable chunk of extra damage at the end of the turn.
An excellent Special Attack stat enables Darkrai to deal plenty of damage with its special attacks, too. It can also learn Nasty Plot to further increase this stat. Darkrai can learn the reliable and powerful Dark Pulse, as well as Sludge Bomb, Thunderbolt, and Ice Beam. Try giving Darkrai a Life Orb to increase its damage, a Focus Sash to use Dark Void more safely, or a Choice Scarf to get Dark Void off quickly in Double or Triple Battles.
Darkrai will be available from 1st to 24th May, so there's not long to wait.