Absolutely nothing has been revealed ABOUT Marshadow, mind - just its name and appearance. Apparently, further details - including distribution details - will be officially revealed "later".
The trailer in question, by the way, which also features Marshadow, can be viewed here:
Well, that was a thing. Who are these people? What are a Piplup and an Incineroar doing in Kanto? How did Marshadow retcon its way into Ash's entirely-forgotten Ho-Oh adventure? Why does Ash have such a selective memory? Your guess is as good as ours. I suspect we'll know when the movie is released!