You’ll emerge in a large open space. Make your way north to discover a massive impact crater–and waiting at the heart of the crater, the mysterious Pokémon that attacked you at the Altar of the Sunne/Moone. If you want, approach it to try and capture it.
In its weakened state, Necrozma is a lot less threatening, but it’s never wise to underestimate a Legendary Pokémon. Compared to its other forms, it has slightly weaker offensive and defensive stats. However, these stats are still well above average, and its Special Attack is still fairly monstrous, enabling it to hit pretty dang hard.
For attacks, it can use Photon Geyser to deal heavy STAB damage, Night Slash to hurt Ghosts, Power Gem for all other purposes (like annoying Incineroar), and Iron Defense to greatly boost its Defense. Its Prism Armor Ability prevents critical hits. Basically very similar to previous forms, but now with a defensive option.
Because it’s a pure Psychic-type, Dark-types are ideal for dealing heavy damage and resisting Necrozma’s attacks. Otherwise, if you’re paranoid of knocking out Necrozma with super-effective attacks, you could opt for a Steel-type instead. They also resist Necrozma’s attacks, although they’re not immune to Photon Geyser.
When it comes to the actual catching, you don’t need to worry. For a Legendary Pokémon, Necrozma is incredibly easy to catch, presumably because you weakened it so much during your battle atop Megalo Tower. We’d be very surprised if your first Ultra Ball doesn’t ensnare it–and definitely don’t bother using your Master Ball!
In fact, if you can get Necrozma’s HP low enough and cause paralysis, etc., you could even try catching it with one of the fancier balls, like the Luxury Ball or Dusk Ball. Or if you’re really plucky, you could use the Beast Balls that Lusamine gave you or that you purchased from Aether Paradise. You may want to use Roto Catch for this though.
Unlocking Necrozma’s Power
Once you’ve gotten Necrozma on your side, you’ll receive the Ultranecrozium Z that it was holding. Afterwards, Colress will appear to congratulate you. He’ll also give you the N-Solarizer and the N-Lunarizer that he developed, which are used to fuse Necrozma with Solgaleo and Lunala respectively.
Fusing works very similar to Kyurem in Pokémon Black 2 and White 2. After choosing the N-Solarizer or N-Lunarizer, you must select Necrozma in your party and then Solgaleo or Lunala in your party. Necrozma will then absorb Solgaleo or Lunala and become Dusk Mane Necrozma or Dawn Wings Necrozma.
Solgaleo and Lunala aren’t lost forever though. So long as you have an empty space in your party, select the N-Solarizer or N-Lunarizer again and point it at the fused Necrozma to split it back into two Pokémon again. Now for the real fun! When Necrozma is fused, if it’s holding Ultranecrozium Z, a new option will appear in battle: Ultra Burst.
After selecting “Ultra Burst” and a move, the fused Necrozma will transform into Ultra Necrozma before that round of battle begins. It will then remain in this form until it gets KO’d or the battle ends. Basically, it works like Mega Evolution, but does not occupy a Mega Evolution “slot”, so you can still Mega Evolve a Pokémon.
Last, but not least, during subsequent turns, Ultra Necrozma will have access to a unique Z-Move, “Light That Burns The Sky”, via Photon Geyser. Yup, you heard right. Ultranecrozium Z allows Necrozma to essentially Mega Evolve and use a Z-Move. This level of sheer brokenness is only surpassed by Mega Rayquaza.
Having captured Necrozma or otherwise, travel east from the impact crater. About halfway up the slope, there’s a narrow path towards the north. Squeeze through the path and head all the way west at the end. Grab the X Sp. Def that must be freezing outside in the cold, then continue east up the slope, towards the exit to the next area.