From Bulbapedia, the community-driven Pokémon encyclopedia.
Poipole's world (Japanese: ベベノムの世界 Bevenom's world) is an anime-exclusive location found in Ultra Space. It debuted in The Shape of Love to Come!. It is the home to a horde of wild Poipole, including the Poipole that belonged to Ash; the horde is led by a Naganadel.
Poipole's world was once filled by the light of Ultra Necrozma, which made the world a lush and green planet. Necrozma would often protect the Poipole and Naganadel. One day, a meteorite came crashing down, and in order to protect the world from its impact, Necrozma collided with it. In the process of destroying the meteorite, it turned into a different form with a black appearance and fell into a crevasse. After falling, the change in Necrozma's form caused it to be encased in a spiky and crystallized cocoon, which subsequently turned the green and bright world into a desolate and dark place. The Poipole and Naganadel huddled together and attempted to take in the remnants of light. A sole Poipole later left the world, attempting to find a better place to live. Soon after, a Solgaleo nicknamed Nebby and a Lunala came to the world through an Ultra Wormhole, causing Necrozma to awaken and begin attacking them in an attempt to absorb their light.
Poipole's world first appeared in The Shape of Love to Come!, during Poipole's dream, where it was seen talking with the ailing Naganadel. The Ultra Guardians then came to the world through an Ultra Wormhole produced by Lunala in The Prism Between Light and Darkness!, in pursuit of Necrozma, who had taken over the body of Nebby. The Guardians were attacked by wild Poipole, but soon relented when Ash's Poipole talked to them. Naganadel then appeared from the hive interior to check on the commotion, but soon collapsed from weakness. Everyone went into the hive's interior, where, after being fed a batch of Sitrus Berries, Naganadel felt well enough to talk. Through telepathy, Naganadel relayed the history of its world to the Ultra Guardians. It then pleaded the Ultra Guardians to allow Necrozma to absorb Solgaleo and Lunala's light so that their world can be saved.
Their discussion was interrupted by Necrozma and Lunala, who had begun their fight anew in front of the Poipole hive. Gladion attempted to intervene and help Lunala, but was eventually stopped when Ash and his classmates came up with a plan to help both Necrozma and Nebby. Ash and Pikachu, Kiawe and Marowak, Lana and Popplio, and Gladion and Silvally launched their Z-Moves at Necrozma to share their Ultra Aura with it. However, the plan backfired, and only succeeded in making Dusk Mane Necrozma stronger. Just then, the Ultra Guardians home base established communications with the team. After Lusamine, Professor Burnet, and Wicke explained their findings, Lusamine coordinated with the Island Kahunas to channel the Ultra Aura of every person and Pokémon in Alola. Combined with the Ultra Aura from Nebby and Lunala's respective Z-Moves, Necrozma and Poipole's world were restored to their former glory.
After the battle, Ash and Pikachu shared a tearful goodbye with Poipole, who chose to remain home with its kind.
Poipole's home world without Necrozma's light
The interior of Poipole's nest
Pokémon seen in Poipole's world