In addition to its Dusk Mane and Dawn Wings forms, Necrozma has a third form in Pokémon Ultra Sun and Ultra Moon. Continue reading if you don’t mind seeing potentially massive spoilers. Necrozma’s third form is officially known as Ultra Necrozma, a Psychic/Dragon-type Pokémon. The official artwork and Pokédex entries for this new form are as follows:
Pokémon Ultra Sun: This is its form when it has absorbed overwhelming light energy. It fires laser beams from all over its body.
Pokémon Ultra Moon: The light pouring out from all over its body affects living things and nature, impacting them in various ways.