Necrozma

"Run away... that thing... is a monster!"

- Lusamine about Necrozma.

Necrozma is the main antagonist of the videogame Pokémon Ultra Sun and Ultra Moon. It is a Psychic-type Legendary Pokémon introduced in Pokémon Sun and Moon.

Sun and Moon
In the post-story, after obtaining every Ultra-Beast released into Alola from Ultra Space by Lusamine and Nihilego, Looker mentions a black figure he saw flying around Melemele Island, but shrugs it off since all the Ultra Beasts have been captured. However, visiting Ten Carat Hill will trigger a battle against Necrozma, where the player can either capture or defeat it.

Ultra Sun and Ultra Moon
Necrozma is an entity that wanders through Ultra Space, snacking on the light released from dimensions. One day, it found its way into the Alola region, but the Guardian Deities managed to defeat it and banish it back into Ultra Space. Later, it came to Ultra Megalopolis and stole the light, but the people managed to restrain it. As its strength kept growing, the Ultra Recon Squad from Ultra Megalopolis explored other worlds to find and stop it.

When Lusamine opened the Ultra Wormholes to locate and capture Necrozma, the entity kicked her and Guzma out and began to wreck havoc, but is attacked by Nebby in the form of Solageo or Lunala depending on the player's game. Necrozma manages to defeat Nebby and forcefully merges its way into Nebby, becoming Dusk Mane Necrozma or Dawn Wings Necrozma, battling the player to test its new found power. After defeat, it retreats into Ultra Space.

Afterwards, the player is acquired by the Ultra Recon Squad to chase after Necrozma on Lunala in Ultra Sun or Solgaleo in Ultra Moon. The player is taken to Ultra Megalopolis, where reaching the top of Megalo Tower, they will find Necrozma reaching its Ultra Burst and becoming Ultra Necrozma, its ultimate form. It will challenge the player and their Pokémon, where all of its stats are boosted by aura.

Upon defeating Ultra Necrozma, it releases Nebby and falls through the floor into another world, without specification on where it went. On the way to the Pokémon League on Mount Lanakila, the player can make a detour and discover Necrozma in a crater on the mountain, exhausted from its loss of light. The player is given the option to share some light from their Z-Ring to Necrozma, and should they accept, Necrozma will become well enough to battle, where the player can be able to capture it. After defeat, the player will be rewarded the Ultranecrozium Z and obtain either a Solarizer or Lunarizer from Colress to merge Necrozma to Solgaleo or Lunala.

Trivia

 * Despite coming from Ultra Space, Necrozma is not related to the Ultra Beasts; however, the International Police have no records of it ever emerging from an Ultra Wormhole so its classification is still uncertain.
 * Necrozma is the 800th Pokémon in the National Pokédex.
 * Necrozma shares its battle theme with Solgaleo and Lunala, even though it shares no connections with the two.
 * Necrozma is the most evil Pokémon to exist in the game franchise besides Darkrai from the Mystery Dungeon series.