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Illusion Attribute Magic was not a magic that originated from Kritias, although illusions could still be created through Water and Light Attribute magic, the mages that could employ them successfully were incredibly few, making it a vastly unique and new type of magic, and one of Kireina's specializations.
Since she was reborn in Genesis as a Caterpillar that she was given such power the moment she reached her third stage as a Dark Moon Illusion Butterfly, acquiring the power to generate illusions and manipulate shadows.
In Genesis, the development of Magic is simply outstanding compared to Kritias, to the point that new types of magic surge one after another, divergent of the original elements, or completely new concept brought by the vast amount of Gods that exist in such a world.