I just received Marshadow and I was wondering if there was an in-game event for him, is there?
How did you manage to obtain one? Both Serebii and Bulbapedia confirm that Marshadow is a currently unobtainable event Pokemon. Bulbapedia hasn't got a sprite up at all, and Serebii is only using an unofficial sprite, so your Marshadow is probably hacked by some really good hacker.
Hope I helped. :)