https://www.polygon.com/2017/10/5/16427084/pokemon-ultra-sun-ultra-moon-trailer-story-necrozma

The Pokémon Company has mostly kept details about November’s Pokémon Ultra Sun and Ultra Moon, which follow last year’s Pokémon Sun and Moon, close to the vest. The game’s latest trailer changes that up, introducing us to some new Ultra Beasts, and a city and storyline that looks a whole lot darker than our last visit to the Alola region.

Ultra Sun and Ultra Moon will take players to a new realm, sending them through what’s called an “ultra wormhole” to find a parallel universe to Alola. Once at the capital city of Ultra Megopolis, it’s clear that something’s off: Necrozma, the most mysterious legendary from the previous seventh-gen titles, has darkened the entire Pokémon world, robbing it of its light sources.