The final Pokemon that you can catch in Alola only appears after you've completed the UB Taskforce sidequest. At the end of that quest, you'll receive a subtle hint that there's more than meets the eye. However you won't learn much more than that.
Thankfully, we're one step ahead of the task. Head to Ten Carat Hill on Melemele Island; if for some reason you've never been, it's west from Professor Kukui's lab. Inside, make your way to the end, towards the Farthest Hollow.
There, if you roam around the grass, a mysterious creature may attack you. You'll know you've done it right when the normal encounter music doesn't play. As with the UBs, you can use the Repel trick, so long as your lead Pokemon is above Level 10 or so and below (or equal to) Level 75.
Not a UB, although it seems similar. Rather, it appears to be the third part of a Legendary trio alongside Solgaleo and Lunala.
Unlike those two though, it's not particular strong. Still, at Level 75, it can be pretty formidable. Its signature move Prismatic Laser does a lot of damage, but causes Necrozma to recharge afterwards, just like Hyper Beam etc.
Its other attack is Wring Out, which does more damage against high HP foes. When it's not attacking, it can use Stealth Rocks to inflict damage to Pokemon that switch in and Iron Defence to rapidly boost its Defence stat.
A Dark or Ghost-type Pokemon is recommended to withstand Prismatic Laser and Wring Out respectively. You will have to be cautious about accidentally KOing it with super-effective damage though.
Otherwise, apply a status affliction as soon as you can. Paralysis is good since it lasts for the entire battle, but Sleep can give you a better catch rate.
My Hand Slipped...
If you make a mistake and KO Necrozma, for example with a badly-timed critical hit, it's not the end of the world. Just like the UBs, the Prism Pokemon will respawn straight away.
That's all for the UBs and Legendary Pokemon... or is it? Actually, there's one more thing that will require a trip to another world.
The Alternate World
This is actually a bit less exciting than you might expect, but still useful knowledge to have. If you head back to the Altar of the Sunne/Moone, where you battled Solgaleo or Lunala, you'll find an eerie distortion towards the far end.
If you have Solgaleo or Lunala in your party and, depending on the current time of day, you can actually pass through the distortion to an alternate world.
- Day-time: Lunala
- Night-time: Solgaleo
We should mention that the requirements have nothing to do with owning Pokemon Sun or Pokemon Moon. Indeed, if you have both Solgaleo and Lunala (you'll probably need to trade for the other), you can pass through the distortion at any time.
On the other side, you'll discover a world that's identical to the one you came from, but with two differences. Firstly, the time of day is shifted by 12 hours. So if it was day time in the original world, it's night time in the alternate world (and vice versa).
Secondly, the following locations change:
- Altar of the Sunne becomes Altar of the Moone
- Lake of the Moone becomes Lake of the Sunne
- Altar of the Moone becomes Altar of the Sunne
- Lake of the Sunne becomes Lake of the Moone
In fact, if you're playing Pokemon Sun, you could say you've traveled from the world of Pokemon Sun to the world of Pokemon Moon--and the other way around if you're playing Pokemon Moon.
However the changes are purely cosmetic and the actual Pokemon you encounter in the original world and the alternate world are exactly the same. That said, the time shifting is real, so you will encounter night time Pokemon during night time in the other world, etc.
This can be immensely useful for catching certain Pokemon or finding certain Zygarde Cells that only appear during the day or night. For example, say it's currently night time, you can head to the alternate world to search for Zygarde Cells that appear during the day without waiting.
Besides this, there's one Pokemon you can only find in the alternate world. If you head to the Lake of the Sunne/Moone in the other world, with Solgaleo (Sun) or Lunala (Moon) in your party, you'll find the Legendary Pokemon Cosmog waiting at the very end.
Just like in the story, Cosmog is completely helpless and needs to be raised to Level 43 before it evolves into Cosmoem, which is just as helpless. After Cosmoem finally reaches Level 53, it will evolve into Solgaleo in Pokemon Sun and Lunala in Pokemon Moon.
To easily level up Cosmog and Cosmoem, we recommend giving it the Lucky Egg from Professor Kukui and using the Exp. Share so it can earn experience without battling. You could also make use of Poke Pelago's Isle Evelup if you prefer a hands-free approach.
By the way, you may think it's a bit redundant to get another Solgaleo in Sun and another Lunala in Moon--and that much is true. What you can do is trade your spare Solgaleo or Lunala to somebody playing the other version. That way both of you can get Solgaleo and Lunala.
You can even go one step further and trade Cosmog or Cosmoem to the other version and have the other player evolve it into Solgaleo or Lunala for you. It's the exact same thing, but you can have Solgaleo and Lunala both with your trainer ID number, which is nice.
Making a Splash
One more thing! This may not be important except for hardcore battlers, but Cosmog can learn Splash, while Solgaleo and Lunala cannot. Although Splash itself is useless, its Z version can boost Attack by 3 stages. That's something to think about... especially for Solgaleo.