Should you return to the Altar of the Sunne/Moone, you can reach a new destination via the same warp hole used to access Ultra Warp Ride and travel to Ultra Megapolis. However you will need Solgaleo or Lunala in your party (they can also be fused into Necrozma) and you must come during the right time of day.
If it’s day-time, you’ll need Lunala’s help. Meanwhile, if it’s night-time, you’ll need Solgaleo’s assistance. Of course, if you have both of them in your party, you don’t need to worry about what time it is. Bring the correct Legendary Pokémon for the time of day and a new option will appear: the reverse world.
Choosing this option will transport you to the Altar of the Moone in Pokémon Ultra Sun and the Altar of the Sunne in Pokémon Ultra Moon. Additionally, the time of day will be the opposite of what it originally was --so if it was night before you entered the warp hole, it will be day when you arrive.
Essentially, the reverse world is the world of the opposite game version… kind of. Only the time of day and two locations are affected. Besides the Altar of the Sunne/Moon being flipped, the Lake of the Sunne/Moone in Ula’ula Island are also be flipped. However, everything else is unchanged, including wild Pokémon.
Still, being able to control whether it’s day or night is pretty cool–and it should help you evolve Pokémon that depend on the time of day more easily. Actually, there is one more thing. While in the reverse world, if you go to the altar at the end of the Lake of the Sunne/Moone, you can encounter a very special Pokémon.
For this to happen, Solgaleo (Ultra Sun) or Lunala (Ultra Moon) must be present in your party, and not fused into Necrozma. If you did it right, Cosmog will appear and you can obtain it simply by interacting with it. At the same time, you’ll receive Lunalium Z in Ultra Sun or Solganium Z in Ultra Moon, filling the gap in your collection.
True to its role in the story, Cosmog is weak and has no attacking moves. Once it reaches Level 43, it’ll evolve into the equally helpless Cosmoen. Finally, if you can get it to Level 53, it will evolve into Solgaleo in Ultra Sun or Lunala in Ultra Moon. We recommend giving it the Lucky Egg from Kukui and using the Exp. Share to train it.
Although it seems redundant to have another Solgaleo or Lunala, what you can do is trade your duplicate Legendary to get the other one. Or you can loan your Cosmog or Cosmoen to somebody playing the opposite version and have them evolve it into the other Legendary, before trading it back to you. Best of both worlds!