After you have helped capture all the Ultra Beasts, then Looker will return to the motel and exclaim that he saw a mysterious black Pokémon flying over Melemele Island. Anabel reassures him that it was just his imagination as all the known Ultra Beasts have been captured, so he shrugs it off. However, if you go to the Farthest Hollow in Ten Carat Hill, you may find the Legendary Pokémon, Necrozma. While appearances show it is akin to Ultra Beasts, it is not so the Beast Ball will not have additional functionality on it