http://www.pokemonbank.com/en-us/news/
"Pokémon Bank has undergone major updates and now works with
Pokémon Sun and
Pokémon Moon, making it easy to transfer Pokémon from multiple copies of
Pokémon Sun and
Pokémon Moon—including the downloadable versions—to a single game! You can also use it to transfer Pokémon you’ve caught in the Nintendo 3DS Virtual Console versions of
Pokémon Red,
Pokémon Blue, and
Pokémon Yellow into your
Pokémon Sun or
Pokémon Moon game. Pokémon from
Pokémon Omega Ruby,
Pokémon Alpha Sapphire,
Pokémon X, and
Pokémon Y can also be brought into
Pokémon Sun and
Pokémon Moon in the same way.
To celebrate this update, all
Pokémon Banksubscribers can get a Mewnium Z, the Z-Crystal that only Mew can use. When you launch the new
Pokémon Bank and receive the gift, a new Wonder Card will appear in your
Pokémon Sun or
Pokémon Moon game. You can use the Mystery Gift function to claim your Mewnium Z.
Note that this gift is for
Pokémon Sun and
Pokémon Moon players only. It is not available in
Pokémon X,
Pokémon Y,
Pokémon Omega Ruby, or
Pokémon Alpha Sapphire. Also, you can get only one Mewnium Z, even if you use
Pokémon Bank with multiple
Pokémon Sun and
Pokémon Moon games. If you have
Pokémon Bank, be sure to use the service by October 2, 2017, to get your Mewnium Z."
So THAT's how you get Mewnium Z.
Hm. Is the Psychic Terrain added before or after the Z-Move? 185 BP off of an OK base 100 attacking stat + Psychic Terrain sounds pretty solid for a nuke.