A few things to share with some of you guys.

--There are a grand total of 18 Z-Stones--one for each type. However, you need to defeat the respective Totem Pokémon in order to win it.

--There are only 4 Totem Pokémon per main island. Solgaleo and Lunala, however, are both version-exclusive Psychic-type representatives in which defeating or capturing them will award you with the Psychic Z-Stone.

--However, there are two secret islands that will house two remaining hidden Z-Stones. Both secret Totem Pokémon happen to be Legendary Pokémon.

--The big building on the artificial island is the Champions League. You can face all of the regional champions you have fought and defeated there in a tournament setting (such as Red, Blue, Lance, Steven, Wallace, Cynthia, etc.).

--The castle behind it, however, is where you can face the four Kahunas as an Alolan Elite 4. Defeating them again earns you the right to face the Champion of Champions.

--Mashadow is Psychic/Dragon. And he is one of the Secret Totem Pokémon.

--Lugia and Ho-Oh are both in the game, but neither one are Totem Pokémon. nor do they have new forms.

--Mega Dragonite is Dragon/Dark.

--Mega Zubat is Ground/Flying.

--Team Skull do end up summoning Solgaleo or Lunala, but unintentionally.

--The final island, surprisingly enough, do not have any trial captains and it is up to the player to figure out your trials for yourself. The only resident on the island is the Kahuna himself.

--Your final objective in the main story is to defeat or capture Solgaleo or Lunala, rather than to challenge the Elite 4. The Alolan "Pokémon League" simply allows you to rematch the 4 Kahunas at their full strength.

--Z-Moves, when used, can trigger transformations on certain Pokémon--mainly the Starters as well as Rockruff's evolution. This is also how you summon Ash-Greninja in the game.

--Zygarde is the ultimate catalyst behind the Z-Stones as well as these strange phenomenon.