According to simulations run on different futures of Ignis and humanity, Revolver says that Lightning's is the only one that leads to mutual destruction.
The summoning conditions are 
at least three Armatos Legio!
Die Beschwörung verlangt
mindestens drei Armatos Legio!
Necesito mínimo tres Armatos Legio.
As condições de invocação são
ao menos três Armatos Legio!