Ultra Megalopolis is a city situated in Ultra Space that can be accessed from Alola through an Ultra Wormhole.
The world had its light stolen by Necrozma. In the center of the city is Megalo Tower, a building that provides artificial light to replace the light Necrozma had taken previously and to act as a prison to keep it from escaping to other worlds.
It is the home of the Ultra Recon Squad. The Ultra Beasts Poipole and its evolution Naganadel originate from the world in which the city is located, and are stated to not only be numerous in number, but also popular enough to be treated as starter Pokémon for its residents.
The people of this world have pale, gray-blue skin due to the lack of natural light in the world after Necrozma stole all of it. The Ultra Recon Squad wear futuristic suits that allow them to travel through Ultra Space. While other people do exist in this world, they are not encountered in-game, merely referenced inside buildings either hiding from Necrozma after it is broken free from captivity or cheering in celebration after it is defeated by the player.
History of Ultra Megapolis
Long time ago, the ancestors of the people of Ultra Megalopois tried to take advantage from the power of Necrozma but the result were catastrophic, Necrozma lost its light and stole the light of this world, condemning the inhabitantis to live in a eternal darkness.
After Necrozma forces Nebby to fuse with it and run away into the Ultra Wormhole, the player character needs to get to Ultra Megalopolis through the Withe Warp Hole. Phyco and Soliera (US) or Dulse and Zossie (UM) are waiting for the player character at the front of Megalo Tower to explain that Necrozma will regain its true power and the other people are isolated for fear of Necrozma’s rage.
If the player character returns to Ultra Megalopolis after defeating Necrozma, the player character can talk with Phyco/Zossie if wanting to change the Pokémon that is used in the Ultra Warp Ride.
Pokémon in Ultra Megapolis
If the player character doesn’t accept the Poipole that Soliera/Dulse offered, they are still going to offer Poipole in Ultra Megalopolis.