From Bulbanews, your community Pokémon newspaper.
To star in 20th movie
Immediately after the release of a new Japanese trailer for the 20th Pokémon movie that briefly showed Marshadow, Marshadow was officially announced.
While fans have known about Marshadow since the launch of Pokémon Sun and Moon due to datamining, this is the first time its existence has been officially acknowledged. Marshadow was the only Pokémon in the game data of Pokémon Sun and Moon that had not yet been officially revealed.
While the reveal trailer gave very little information about Marshadow, information obtained via datamining Pokémon Sun and Moon can be found on its Bulbapedia article.