The Pokémon Company International announced that Marshadow will be distributed to players of Pokémon Sun and Moon via a Serial Code at participating GameStop locations from October 9th, 2017 until October 23rd, 2017.

Make Room for Marshadow in Pokémon Sun and Pokémon Moon

The Mythical Pokémon Marshadow is coming to your Pokémon Sun or Pokémon Moon game with a code available at GameStop.

The mysterious Marshadow will soon be able to step out from the shadows and into your Pokémon Sun or Pokémon Moon game. To get the newly discovered Mythical Pokémon, stop by your local participating GameStop store from October 9–23 and pick up a code that will unlock Marshadow in your game.

As the only known Fighting- and Ghost-type Pokémon, Marshadow brings a unique twist to battle. When it unleashes an attack, its body turns green thanks to its burning fighting spirit. The Pokémon you'll receive is ready for action with its signature Spectral Thief attack, which not only does damage but steals the stat boosts of its target. The power of the Marshadow you can get at GameStop doesn't stop there—it also comes holding its special Z-Crystal Marshadium Z, allowing it to strike hard with its signature Soul-Stealing 7-Star Strike Z-Move!

This first opportunity to get Marshadow won't last long. If you want to get this amazing Pokémon, be sure to visit GameStop from October 9–23. Then get ready to shine in battle with the Mythical Pokémon Marshadow in Pokémon Sun and Pokémon Moon!

Marshadow Details:

Level: 50
Ability: Technician
Held Item: Marshadium Z
Moves: Spectral Thief, Close Combat, Force Palm, Shadow Ball

Source: https://www.pokemon.com/us/pokemon-news/make-room-for-marshadow-in-pokemon-sun-and-pokemon-moon/