Nihilego

Nihilego is a Rock/Poison type Ultra Beast and the true main antagonist of the videogame Pokémon Sun and Moon. It was the one who corrupted Lusamine, and is responsible for the events of the game.

History
Lusamine, the president of the Aether Foundation, and her husband Mohn were researching the existence of Ultra Beast. When Mohn suddenly disappeared one day while researching the beasts, Lusamine became obsessed about the Ultra Beast in a desperate attempt to find her husband. She eventually come across Nihilego, who then corrupted her mind. This cause Lusamine to be obsessed over Nihilego, loving it even more than her own children, to the point that she made her own daughter Lillie dress to resemble Nihilego.

Nihilego is first encountered during the player's first visit to the Aether Foundation. It suddenly appeared before the player, Hau, and Lusamine during their tour, and it then suddenly attacks the player. The player defeats the Ultra Beast, unable to capture it due to the jamming signal in the Aether Foundation that prevents the use of Poke Balls. It quickly fled back to Ultra Space, with Lusamine commenting that it looked like it was suffering and vowing to "Save it and love it".

Later, when the player confronts Lusamine in order to save Lillie and Cosmog, Lusamine forces Cosmog to create an Ultra Wormhole, allowing her to summon Nihilego. This also caused numerous wormholes to appear throughout the Alola Region, and as a result, numerous Ultra Beast also emerge from the wormhole. When the player defeats Lusamine, Nihilego once again retreat to Ultra Space, with Lusamine and Guzma following it.

The player and Lillie were eventually able to revive the legendary Pokemon Solgaleo/Lunala. With the help of their new powerful ally, they head to the Ultra Space in order to rescue Guzma and Lusamine. They encounter Guzma, who was noticeably scared of the Ultra Beast while attempting (and failed) to catch one.

They finally confronts Lusamine, who reveals that she kept her Nihilego in a special Poke Ball known as Beast Balls, which were designed to capture Ultra Beast. She releases her Nihilego from the Beast Ball, then merge with to defeat player. While fused, Lusamine Pokemon not only has an sinister look on their face, but also have their stats boosted. After a long battle, however, Lusamine was once again defeated.

In retaliation, they attempted to attack the player and Lillie themselves, but were attacked by Solgaleo/Lunala. Its attack was strong enough to separate Lusamine and Nihilego, rendering Lusamine unconscious. Suddenly, they are attacked by a group of Nihilego, but they were able to flee thanks to Solgaleo/Lunala.

After the events, it was revealed the Nihilego poisoned Lusamine mind in order to make her care for Nihilego more than anything else. This cause Lillie to journey to Kanto with her mother in order to seek help from Bill, who was once combined with a Pokemon.

After beating the Pokemon League, the player is given a letter asking them to come to the motel on Route 8. There, they are asked to join a secret police unit tasked on eliminating or capturing the Ultra Beast. After the player returns from getting the Beast Balls from Wicke, Looker bursts in, reporting sightings of an Ultra Beast in Wela Volcano and Diglett's Tunnel, and asks the player to hunt it down. This Ultra Beast turns out to be Nihilego, and the player can then catch one for themselves.