How to get Zygarde in Pokemon Ultra Sun?
Step 1:
Go to the altar of the sunne during the day while playing Pokemon Ultra Sun
Step 2:
Interact with the altar to travel to Ultra Megalopolis
Step 3:
Make your way to the Megalo Tower and battle your way up to the top floor to encounter Zygarde
Step 4:
Defeat or catch the Zygarde and add it to your team!