Shiranui pulled out his Pokédex and said to it, "Dexter, cross-reference Ultra Beasts and light."  One result came up and Shiranui accessed it.
"[i]Necrozma, the Prism Pokémon.  Reminiscent of the Ultra Beasts, this life-form, apparently asleep underground, is thought to have come from another world in ancient times.  Light is the source of its energy.  If it isn’t devouring light, impurities build up in it and on it, and Necrozma darkens and stops moving.[/i]"