Red Eye (レッドアイ) is a lifeform that existed during the War with The Legion, a watershed event in Nier's world history.
Red Eyes are red-eyed monsters that are unique to the mindless Legion horde; they possess intelligence, individuality, and mightier fighting capabilities. Their sizes vary from human size to topping buildings, and they can sprout multiple limbs to attack humans. More Legion can be conjured to an area by their will, making them one of humanity's toughest adversaries.
They are thought to be created when a person infected with White Chlorination Syndrome forms a contract with the god of another world. Unlike the common Legion with the single-minded directive to annihilate humanity, the god has a higher influence on Red Eyes.
According to side materials, Replicants can also become Red Eyes. Dead Replicant-Red Eyes manifest an egg-shaped object near their remains.