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Tapus were long lived fairies who supposedly gain powers from the nature on the islands. They also had the biggest dicks in Alola at the time and could kick everyone's ass/ do mystical shit. They were long lived and capricious, so they were worshiped as deities.
A solgaleo and lunala showed up in Alola one day in order to find a safe dimension to fuck in. The Alolans immediately saw how strong they were and named them as deities. The tapus tried to fight them and got their asses kicked. Necrozma showed up to try and rape them one time, but the tapu/ alolans managed to fight it off. After fugging and dumping the baby cosmog on the Alolans/ Tapu, they left.
Necrozma is a bit weird. The people of Ultra Megalopolis respect it and gave it an honorary title. However, they kept it imprisoned in a tower after their ancestors broke. Light, pieces of its body, and Z-power were scattered across the dimensions and it ate the dimensions light in response.
From a famitsu interview, they designed its role in the game to be like a "cultural hero."
>Iwao: The way of illustrating Necrozma is a point we took a lot of care of. How do we fire out his presence as big, we thought a lot of it. So we though adopting the concept of "cultural hero", an outline that captures myths and traditions left on the different places on the world we live in. This means, not an almighty "god", but rather an special entity that gives humans things that make their lives abundant, like fire and crops. It can be attributed to Prometheus from Greek mythology or Maui from the Polynesian lore, however there's also cultural heroes who are two-faced tricksters who like mischief. In this game, we also took the framework of cultural hero for Necrozma, this way we thought we could make it leave a strong impression on the players.