Old Wall

The Old Wall refers to the power to activate or control the giant statue guardians placed around the city of Insomnia, this power is granted by the Lucii/Past Kings and is presumed a last line defense.

During the film Kingsglaive, Regis discusses the Old Wall with his council and decribes himself as being too weak to summon them, this would infer that summoning the Old Wall puts a great strain on the user. We see later on Nyx becoming weak and having a charred body, although it is not clear whether this is a result of the Old Wall or over use of magic in general during his fight with Glauca.