One of several mysterious Ultra Beasts. People on the street report observing those infested by it suddenly becoming violent.
– Pokémon Moon Pokédex
Nihilego is the first revealed Ultra Beast. It was designated as UB-01 Symbiont. It is also the first Ultra Beast you encounter in the game, when you meet it at Aether Paradise. I remember trying to figure out what type it is by hitting it with different moves.
Its typing of Rock/Poison is unique. I can see the Poison influence, since it’s the “Parasite Pokemon”, and it has been seen taking over other beings (like Lusamine), but I’m not sure where the Rock typing comes from…
For some reason, it takes the appearance of a little girl, in particular, it looks rather like Lillie. Nihilego has a story role, in that it takes over Lusamine and you have to fight Lusamine!Nihilego to save Lusamine in the Ultra Beast’s world.
Stat-wise, it has high Sp Def, Sp Atk, and decent Speed; it has low Attack and Defense however.