Having conquered the Vast Poni Canyon, as well as an impromptu island trial, you’ll finally reach the altar of the Legendary Pokémon, where further trials await. As you arrive, Lillie will catch up and heal your Pokémon. Afterwards, head all the way up the giant staircase that’s directly ahead. Do yourself a favor and call Tauros.
At the summit, you’ll emerge atop the altar itself. Lillie will go over to the conspicuous square panel towards the west, and instruct you to stand on the corresponding square panel towards the east. Navigate your way to the panel and when prompted, play the flute in your possession. At this point, you may want to grab some popcorn.
Just when everything is going according to plan, a mysterious Pokémon appears to crash the party.
|Dusk Mane Necrozma (Ultra Sun)||50||Psychic/Steel|
|Dawn Wings Necrozma (Ultra Moon)||50||Psychic/Ghost|
During this battle, you’ll face off against Necrozma that has fused with one of the Legendary Pokémon. If playing Ultra Sun, you will have to fight Dusk Mane Necrozma, which resembles Solgaleo. Meanwhile, Ultra Moon players will encounter Dawn Wings Necrozma, which resembles Lunala.
No matter what form Necrozma has taken, your goal is the same: to knockout Necrozma. Don’t worry about trying to capture Necrozma, because you can’t at this point in time. Compared to the Totem Pokémon previously, this battle should be easier in theory, but this is a Legendary Pokémon we’re talking about.
Dusk Mane Necrozma boasts incredible Attack and generally robust stats, besides its middling Speed. It can use Sunsteel Strike, a powerful Steel-type move that ignores your Pokémon’s Ability, Morning Sun to heal a lot of HP, Psycho Cut to deal Psychic STAB damage, and Night Slash to hurt Ghost-types.
Ironically, Fire-types are the best Pokémon to use here, since they resist Sunsteel Strike. Those who picked Litten–and now have a Incineroar–will be laughing since it’s immune to Psycho Cut and resists Night Slash. Otherwise, Ground and Dark-types are the next best choices, and Ghost-types if they’re prepared to eat a Night Slash.
Stat-wise, Dawn Wings Necrozma is the same as Dusk Mane, but its physical and special stats are swapped, making Special Attack its highest stat. This form can use Moongeist Beam to deal powerful Ghost-type damage and ignore Abilities, Moonlight to heal lots of HP, Psychic for STAB, and Power Gem to surprise Incineroar users.
Dark and Ghost-types are ideal for this Necrozma form, as their attacks deal 4x damage. Obviously Dark-types are better off since they don’t take super-effective damage from Moongeist Beam and are immune to Psychic. Normal-types could be useful since they’re immune to Moongeist Beam, but they’ll need a way to attack.
Successfully drive off Necrozma and it’ll escape back to its home turf. Now you’re back to square one. Fortunately, all hope isn’t lost, as the Ultra Recon Squad arrive on the scene and prove that they can actually be useful for once. You’ll borrow their Legendary Pokémon and use it to enter Ultra Space in search of the world that Necrozma fled to.
For now, your aim is to fly through the balls of energy and travel through one of the pure white warp holes. Succeed and you’ll reach Ultra Megapolis, where Necrozma can be found. If you enter a different warp hole, you’ll receive a message that Necrozma isn’t here. You must then exit this world and speak to the Ultra Recon Squad to try again.