Necrozma has a secret ultimate form called Ultra Necrozma. It's a Psychic/Dragon type with a BST in the 700s, and it's got base stats of 167 for both attack and special attack. Its sprites are here:
So yeah, that's the gen 7 Zygarde equivalent for you.
Beat the main story. Overall found it more satisfying than S/M and I'm actually surprised how much was changed. It felt like each character's own arc wrapped up neatly.
The game is essentially the same for 75% of the time with the notable differences, aside from new Pokemon, being revamped trials and Ultra Recon Squad warning you about Necrozma and being generally useless. Then you fight Lusamine and instead of wanting to open wormholes just because she wants to protect the Pokemon and fight Necrozma herself.
Also Gladion seems genuinely nicer to her since he volunteers to go with Sivally to fight Necrozma instead of her.
Shit happens at the altar, Lusamine and guzma are shat out of the wormhole and Lusamine actually isn't possessed this time. Then you fight Necrozma and beat it, etc. Also Guzma and Hau actually have a conversation on what their goals are after the Necrozma, which I thought was really nice since in S/M Guzma just sort of disappears after the Lusamine demon fight. Also Lillie and Lusamine actually make up and act like a family instead of Lilli leaving while Lusamine is essentially braindead. The credits are good too in the fact it shows what happens to Hau, Lillie, Gladion, Guzma, and you after the elite 4 instead of just focusing on you and Lillie.
Sadly the following were removed from the game:
-The exeguttor island moment with Lillie
-Guzma saying "Ya'll are stupid!"
-Fight with demonbeast Lusamine
-Kukui trolls you at the champion's seat and using the power of malasadas, Hau is the final battle
--You beat Hau right before his goal and make him shed tears for those of you who aren't fond of him
I havent gotten too far, just cleared the first trial. It's reminded me how good Sun/Moon was because I still love the Alola region and its wacky everything.
my final team will have Decidueye and Golisopod, but I'm really unsure who else I'll end up using so right now im just enjoying using whatever I catch. Current team is a mess of Pokemon ive never really used before but caught because theyre cool to have so early in the game and just some other randos:
Imho, keep Zorua and Crabrawler. This is the perfect duo to cheese the shit out of the fight with the main evil Pokemon (pretty obvious who it is, but spoilers), which is actually very tough if you're not prepared.
My final team was Primarina, Aegislash, Zoroark, electric Oricorio, Incineroar, and Crabominable.
Completed the main quest and the Rainbow Rocket episode.
I said this about the original SM and I'll say it now, it's nice to finally have some challenge in Pokemon for once. The trials, especially the electric one, required patience and strategy. You can't just over level your starter and clear the game. It is pretty similar to SM, but more streamlined and with better QoL. From the Aether Foundation onward the story takes new twists and turns, most of them appreciated, though as Gamefreak pointed out in his post, some stuff is missing, much to the detriment of the story.
The Rainbow Rocket story segment though is pure fan service. I had a lot of fun with it, even if the level jump is a bit crazy. My entire team was about Level 70 for the final battle and I walked away with only one Pokemon left standing, and he was poisoned with 2 HP remaining. It was great fun to see all the callbacks to the past.
All in all, if you liked SM, you'll like this. Whether that's worth $40 is up to you.
Now it's time to hunt UBs and Legendaries, and then transfer my living dex via Bank.
Oh yeah, Necrozma is no joke in this game. Never have I felt so out of my league in a Pokémon battle. As you should when up against a legendary like that.