This is Zygarde’s 100% form. It has enough power to overwhelm even Xerneas or Yveltal.
– Pokémon Moon Pokédex
Zygarde’s Complete forme was introduced in Sun/Moon, along with a 10% forme (its XY appearance is dubbed as the 50% forme). Zygarde takes this form when it has gathered all of its cells. It looks like a giant robot.
In the game, either a 10% forme or a 50% forme Zygarde with the ability Power Construct changes to Complete forme when its HP falls below half (at the start of the turn). In Sun/Moon, you can obtain a Zygarde with this ability by gathering all the Zygarde cells. In Ultra Sun/Moon, the process is much easier; you just have to catch the Zygarde from that game, and talk to Sina and Dexio.
It has base stat total of 708. It has a lot HP, but its other stats are similar to 50% Zygarde.
Zygarde has the highest number of exclusive moves, with 4 (which imo is too many!)