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It's stupid how GameFreak sets up great backstories to their legendaries but never reflect on the cool things.
>The OG Solgaleo and Lunala made Cosmogs and Cosmoems to brighten the universe and use wormholes to travel through the universe and time to get places faster.
>Necrozma was created to watch over the darkness of space and share light with the world, but grew greedy and ate Cosmogs/Cosmoems to harness light for itself.
>OG Solgaleo and OG Lunala fight it and one gets fused to it.
>Becomes Ultra Necrozma, creates Ultra Beasts to replace Pokemon like Mewtwo tried doing in the movie.
>Other Legendary + Zygarde + Alolan Guardians fight it. (Implied in the dialogue of Kukui during the beginning of USUM talking about the people and Pokemon of Alola taking back the light in ancient times)
>OG Solgaleo and OG Lunala die, Necrozma does Ultra Burst, Nebby is the last Cosmog that hides on Earth for centuries.
>Necrozma and the UBs get sealed up in Ultra Space forever for their violence like Giratina. Guardians take the Z-Crystals from Ultra Necrozma and give them to special trainers to help them in need of the time when Necrozma may return.
>Lusamine and Mohn study wormholes to figure out what they are, Mohn disappears, Lusamine gets fascinated by Wormholes more.
>Lusamine finds Nihilego, gets obsessed with it, goes insane from it.
>Opens up wormholes, releases all the UBs AND Necrozma into the real world in the process.
>Gets rehabilitated by Bill in Kanto.
>It is not canon that we ever were supposed to catch or defeat Necrozma in SM, and Ultra Ruins is essentially an alternate timeline in SM.
>Ultra Ruins also implies that you (the player) left suddenly, and everything went to ruin.
>Ignoring the fact that Ultra Wormholes take you far away in space (measured in light years), they're implied to have taken you so far that you go into different realities/parts in time.