As if Mewtwo wasn't powerful enough, the psychic pokémon has a new form in the fighting game Pokken Tournament. Have a look at Dark Mewtwo above.
Dark Mewtwo is a new boss for the game, currently available only in Japanese arcades. Evidently the series updated recently, adding in this evolution. It's uncertain whether this evolution is for Pokkén Tournament only or if we're looking at a canonical new form for the main games as well.
Dark Mewtwo is ultimately beaten here by Gengar. For more, we saw Gengar in action way back in April. Pokkén Tournament comes to the west with a Wii U port launching world wide next spring. The arcade restaurant chain Dave & Busters also said this summer it would be testing out Pokkén Tournament cabinets.