Generation Vii
Power Construct
Other Cells gather to aid when its HP becomes half or less. Then the Pokémon changes its form to Complete Forme.
Detailed Effect
Transforms 10% or 50% Zygarde into Complete Forme when its HP is below 50%. This ability cannot be copied or replaced. This ability only takes effect for Zygarde.
Pokémon With Power Construct
3 canonical Pokémon forms can have this Ability.
Zygarde 10 Power Construct 
Zygarde 50 Power Construct 
Zygarde Complete