https://www.gamespot.com/articles/free-mythical-pokemon-for-sun-and-moon-coming-soon/1100-6452931/

Nintendo has announced another distribution for Pokemon Sun and Moon. This time, players will have their first chance to add the recently revealed Mythical Pokemon Marshadow to their teams.

Beginning October 9, players in the US can visit a participating GameStop store to pick up a free code for Marshadow, making this the only official way to obtain the Mythical Pokemon so far. The dual Fighting/Ghost-type monster will be level 50 and comes equipped with its own unique Z-Crystal, Marshadium Z, which lets it unleash the Soul-Stealing 7-Star Strike Z-Move during battle. It also knows the following attacks: