Iskandur was known as having a pretty op ultimate. I can't remember where I read it but apparently he is the only one who can create such a large reality marble for such a long time.
TYPE-MOON Wiki said: [Ionioi Hetairoi: Army of the King] is an irregular Reality Marble. Normally only a magus or something beyond normal reality like Demons are capable of materializing the environment inside their minds, and as Alexander is neither, reproducing it alone is impossible for him. It comes about through the combined efforts of his personal guard, who vowed allegiance to him after many victories and defeats, even after death and becoming Heroic Spirits as well ... Once activated, it is maintained by the collective prana of the entire army.
