Photon Geyser is a new Psychic-typespecial move that only Necrozma can learn. Necrozma can learn this move regardless of form. This move compares the user's Attack and Special Attack stats, dealing damage to the opponent using the higher stat.
Both Dusk Mane Necrozma and Dawn Wings Necrozma have the Ability Prism Armor. Dusk Mane Necrozma has a higher Attack stat than Solgaleo (which has 137 base Attack) while Dawn Wings Necrozma has a higher Special Attack stat than Lunala (which has 137 base Special Attack).
In Pokémon Ultra Sun and Ultra Moon, the player can deepen their bond with the Rotom Dex to get items called Rotom Powers via the Roto Loto feature (known as Rotopon in Japanese). The player can use these special items from their Bag at any time to apply an effect for a set period of time, similarly to Pass Powers and O-Powers.
Some Rotom Powers include:
Roto Hatch causes the player's Eggs to hatch faster.
Roto Bargain decreases the price of items at Poké Marts.
Roto Prize Money presumably increases the amount of prize money the player earns.
Roto Exp. Points boosts the experience earned by the player's Pokémon.
Roto Friendship presumably boosts the amount the friendship of their Pokémon is increased.
Roto Encounter helps the player run into high-level wild Pokémon more often.
Roto Stealth presumably decreases the encounter rate of wild Pokémon.
Roto HP Restore presumably restores a Pokémon's HP.
Roto Capture makes it easier to catch Pokémon.
If the player is close enough with the Rotom Dex, it will use a special power called Rotom's Z-Power, which allows the player to use a second Z-Move in battle.