Necrozma

Necrozma is an ancient Psychic-type Pokémon species rumoured to be found near the Alolan islands. Though it doesn't evolve, Necrozma has three different alternate forms.

Ultra Necrozma has its own Z-Crystal which is called Ultranecrozium Z.

Overview
While in its average state, Necrozma is described as a large black prism-like humanoid with four limbs. When possessing Solgaleo or Lunala, it takes on a Dusk Mane or a Dawn Wings and it also has an Ultra appearance.