Nihilego

Nihilego is a species of Rock/Poison type Ultra Beast Pokémon. Lusamine's Nihilego serves as the Bigger Bad of the video game Pokémon Sun and Moon. It was the one who corrupted Lusamine, and is responsible for the events of the game.

Personality and traits
Like any other Pokémon, Nihilego is not a truly malevolent creature, and the Pokédex entry brings up the fact that it is possibly non-sentient. Like other Ultra Beasts, Nihilego in the normal world become panicked and start attacking people affected by Ultra Wormholes in an attempt to return to their home, but Nihlego have a unique trait that makes them one of the most dangerous types of Ultra Beasts; their venom, which decreases inhibition of their victims while drawing out their latent power. The ultimate goal of this is to drive hosts into attacking and destroying any potential threats to the Nihilego. When Lusamine was constantly affected by Nihilego venom, her preexisting obsession with the Ultra Beasts became outright hostile to anyone who may threaten them, and amplified her negative qualities into borderline sociopathy. Nihilego do not directly control their hosts, however.

History
Lusamine, the president of the Aether Foundation, and her husband Mohn were researching the existence of Ultra Beasts. When Mohn suddenly disappeared one day while researching the beasts, Lusamine became obsessed with them in a desperate attempt to find her husband. She eventually came across the Nihilego, which then afflicted her with their distinct venom. This caused 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.

A Nihilego is first encountered during the player's first visit to the Aether Foundation. It suddenly appears before the player, Hau, and Lusamine during their tour, and proceeds to attack 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 Poké 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, several other Ultra Beasts emerge from the wormholes. When the player defeats Lusamine, Nihilego once again retreats to Ultra Space, with Lusamine and Guzma following it.

The player and Lillie were eventually able to revive the legendary Pokémon 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 Beasts, after attempting (and failing) to catch one.

They finally confront Lusamine in the midst of a swarm of Nihilego, who reveals that she kept her Nihilego in a special Poké Ball known as Beast Balls, which were designed to capture Ultra Beast. She releases her Nihilego from the Beast Ball, then merges with it to defeat player. While fused, Lusamine's Pokémon not only have a sinister look on their faces, 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 several other Nihilego, but they were able to flee thanks to Solgaleo/Lunala.

After the events, it was revealed that Nihilego's poison affected more than just Lusamine's mind, also draining her physical energy after the fusion with Nihilego. This causes Lillie to journey to Kanto with her mother in order to seek help from Bill, who was also once fused with a Pokémon.

After beating the Pokémon 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 Beasts. 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, though likely not the same one, and the player can then catch one for themselves.

In the anime
Nihilego debuted in a flashback in SM047, in which Gladion revealed to Ash that Lillie was once attacked by one when she was younger, resulting in her present fear of touching Pokémon. In SM049, during the flashback of when Nihilego attacked Lillie, Gladion's Silvally arrived and saved her.

In SM050, it was released again after Faba used Nebby's power to open up the Ultra Wormhole. Ash, Gladion, and Lusamine battled Nihilego to protect Lillie from getting attacked, but none prevailed. Just as Nihilego was about to attack Gladion, Lusamine pushed her son out of harm's way and allowed herself to be taken instead. Then Nihilego carried Lusamine into the Ultra Wormhole, leaving Lillie and Gladion behind.

In SM053, it is later revealed that after Nihilego abducted Lusamine, it fused with her and transformed her into a monster, which resulted in putting her and her Pokémon under its control.

Trivia

 * Nihilego is the only Rock-type Pokémon that cannot learn the move Rock Slide.
 * Nihilego may be based on the jellyfish. It may also draw inspiration from, a class of microscopic Cnidarian parasites, particularly the Myxobolus cerebralis that are known to infect "whirling disease" in various fish species. The disease causes the fish to develop bone and neurological abnormalities that cause them to swim in a corkscrew-like "whirling" motion.
 * The fusion form of Lusamine and Nihilego may be based on, a species of jellyfish that is biologically immortal and can revert to a young, immature state if it is too old or stressed.