This Necrozma, also known as the Blinding One, is a Psychic-type Pokémon that appears in the anime. It was thought to be a major antagonist around the middle episode of Sun & Moon - Ultra Adventures, making it more like an anti-villain/fallen hero due to result of losing its light.
In ancient times, Necrozma came to Earth and shared its light with the planet, but caused it to lose its light and begin to assume a weak and dark form, but Solgaleo and Lunala shared their light with it and the Legendary Pokémon was able to resume it's original form and return to its home by the Ultra Wormhole.
At some point, Necrozma fell asleep again, losing its light once more, reassuming the prism shape and getting trapped in a giant crystal in Poipole's homeworld after destroying a meteorite to protect the said realm. Without Necrozma and its light along with the absences of Solgaleo and Lunala, Poipole’s homeworld which was once filled with flowers, turned a desert shrouded in darkness, especially cause the stamina Poipole pack’s leader Naganadel weakened as it grew older.
Many years later in sometimes time after one of the Poipole who manage to find an Ultra Wormhole is now under care of a human named Ash Ketchum, Necrozma woke up from its sleep after sensing Nebby and Lunala when they visited its home. It started to chase them through the Ultra Wormhole and attacked Solgaleo. It later tried to absorb Lunala and followed her to the Altar of the Sunne, where it first encountered Lusamine, Professor Burnet, Wicke, Faba and the Ultra Guardians.
Necrozma and Lunala continued fighting for Alola, by their presence by the region, it is absorbing the present Ultra Aura, weakening the adults.
Even with Lunala receiving support from the Ultra Guardians and a little help from Tapu Koko, Necrozma was able to absorb Lunala and become Dawn Wings Necrozma. But after Nebby appeared and fought against the fused Necrozma, it was forced to disband Lunala. Even after Team Rocket's interference and weakening Nebby, Necrozma absorbs it and becomes Dusk Mane Necrozma before escaping to Ultra Space through the Ultra Wormhole.
After the Ultra Guardians and Lunala arrive at Poipole's homeworld, Necrozma battles against Lunala and Gladion's Silvally. Ash, Lana, Gladion and Kiawe transfer the power of their Z-Moves to Necrozma in the hopes of freeing Nebby and returning Necrozma back to its original form. But instead, Necrozma absorbed the Z-Moves and their efforts only made it stronger. Necrozma goes to attack the group, but was held off by Lunala. The Ultra Guardians learned from Professor Burnet that according to legend, Solgaleo, Lunala, and the people and Pokémon of Alola shared their Z-Power with the Blinding One when it lost its light. So, the Ultra Guardians as well as with the combined Z-Power of everyone in Alola as well as the Team Rocket Trio, the Guardian Deities, and the Island Kahunas, Necrozma gained enough light to free Nebby from its grasp. But despite their successful attempt, Necrozma became angry at their interference and attacked the Ultra Guardians in a blind rage. Hala, Olivia, and Nanu save the Ultra Guardians by transferring their Z-Power to Necrozma. But Necrozma wasn't giving up just yet. Necrozma then goes after Nebby and Lunala again, only to be held off by Gladion and Silvally. With Necrozma distracted, Lunala gave Gladion the Lunalium Z and Nebby recovers. Just as Necrozma was about to attack again, Ash and Gladion combined Nebby and Lunala's Z-Moves to help Necrozma provide enough Ultra Aura to return into its heroic Ultra form. Necrozma then uses its power to restore Poipole's homeworld to its previous majesty and disappears just as the solar eclipse ended. Afterwards, Necrozma appeared in the skies above Alola before dispersing into light.