The official Pokémon website finally gave us some details on their latest addition to Pokémon Sun and Moon, Marshadow. There's no official date yet as to when the new legendary creature will start appearing, but they were more than happy to give us some backstory and status on it before it arrives.
"Known as the Gloomdweller Pokémon, the Mythical Pokémon Marshadow conceals itself in the shadows. It never appears before humans, so its very existence is the stuff of myth. Marshadow sinks into the shadows of others, and is able to copy their movements and power. Marshadow is a very cautious Pokémon. It sinks into shadows and observes what is going on around it in such a way that no one else will notice it. This may be the reason why is so rarely seen. Among the numerous Pokémon discovered to date, Marshadow is remarkably the first to combine Fighting type and Ghost type. Even regarding its types, Marshadow is an extremely rare Pokémon. Fighting-type Pokémon are usually ill-suited in battle against Psychic-type Pokémon, but now you'll be able to take advantage of Marshadow's Ghost type to battle effectively!"
You can find more info about Marshadow's special moves and more at here as we patiently await a release date.