I think Necrozma works the same way this box did in Sun and Moon, and this also could explain why it needs Nebby. Necrozma trys to force Nebby once more to evolve, an stage of evolution never before seen in the Pokémon world, even suprassing Mega Evolution. That weird being of Light we see on the box cover isn't Necrozma, it is in fact 'Ultra' Nebby