Pokemon Solar Light And Lunar Dark Wiki -

According to the Pokémon Ultra Sun and Ultra Moon wiki, Solgaleo and Lunala are said to have a deep connection to Necrozma. In fact, Necrozma is said to have the power to merge with Solgaleo and Lunala, creating two new, powerful forms: Dusk Mane Necrozma and Dawn Wings Necrozma.

Here's an interesting article:

Necrozma, the Psychic-type Legendary Pokémon, is said to be the Dwarf of the Stars. Its name is derived from the Greek word "nekros," meaning death, and "zoe," meaning life. Necrozma has the ability to absorb the light of the stars and use it to fuel its powerful psychic attacks. pokemon solar light and lunar dark wiki