Nihilego

Nihilego (ウツロイド) (code named UB-01 Symbiont) is a /-type Pokémon introduced in Generation VII. It is an Ultra Beast, one of several species of Pokémon native to an alien dimension.

Natural abilities
As implied by its title of Symbiont, Nihilego is specialized towards influencing other Pokemon to protect itself and its nests. It generates a neurotoxin that simultaneously gives those injected with it great power while reducing their inhibitions and sending them into an agitated, often violent state. The ultimate goal of this is for the infected Pokemon to attack and destroy any natural competitors for itself and the Nihilego. It is heavily implied that the violent behavior of other Ultra Beasts is due to acting under the influence of Nihilego venom, and that it is a factor in Lusamine's insanity and hatred of everything she does not feel she can control.

Evolution
Nihilego doesn't evolve.

Trivia

 * As Nihilego is said to be the one who actually caused Lusamine to become evil, Nihilego is the first Pokémon without a trainer who is an antagonist.
 * It is also the first Pokémon to actually be fought as a trainer in the games after fusing with Lusamine.
 * Lillie's dress is based on Nihilego's appearance, as per request of her mother, Lusamine