Altar of the Moone (UltraMoon)

Much of the first part of Poni Island is the same as Pokemon Sun, so I'll just go over it briefly.

As Lillie and I arrive in the Seafolk Village, Mina comes up and introduces herself. She informs us that the island doesn't have a Grand Kahuna, but that we should go and meet Hapu at the Ruins of Hope. We pop over there real quick to see Tapu Fini make Hapu the Island's Kahuna, then its back to the Seafolk Village before going to Exeggutor Island.

On the island, we defend the Exeggutor from a plague of Pinsir before getting the Moone Flute and heading to the Poni Canyon. Team Skull is in the way, we scare them off and Plumeria gives us the Poisinium-Z.

At the end of the Canyon and the trial for the Dragonium-Z, Primarina has it handled until the Totem Kommo-o summons an ally Scizor. From their Greninja and Incineroar are able to finish the Totem Pokemon and its ally.

Passing the trial and climbing the 87 miles of stairs, we arrive at the Altar of the Moone. Lillie and I play our respective flutes. Nebby evolves into Lunala. A wormhole opens up, and Guzma and Lusamine fall through, traumatized by their ordeal. However, they have not come through alone...

Necrozma immediately attacks Nebby, and despite putting up a valiant effort, is absorbed into the light stealing Pokemon...

Uh oh...

It attacks, and Incineroar is able to scare it off. However, it retreats into the wormhole. The Ultra Recon Squad shows up and gives me their Solgaleo to chase after it. I hop on its back and follow it to Ultra Megalopolis. 

I meet the other members of the Recon Squad, and they tell me that Necrozma is at the top of Megalo Tower. After climbing more stairs, at the top of the tower, it awaits me...

I decided to start this battle with Decidueye's Sinister Arrow Raid, which took a pretty sizeable chunk out of Necrozma's HP before being defeated, and then Incineroar, Primarina, Jolteon, and Empoleon managed to chip away at it and finally defeat it. 

After defeating Necrozma, it vanishes and the Ultra Recon Squad is thankful, and give me a Poipole. Upon returning to the regular world, we return to the Seafolk Village, where there is still some business to attend to.