Casio wasn't being pretentious when he said that. He merely wanted to find a subject for his experiment. Because, from the information in Bai Kui's mind about Illusion Arts, he discovered that he seemed to be able to use Bai Kui's Phantom Kui ability in a special way.
Illusionists are a group of illusion users. Their abilities are manifold, but the essence is the same. The Illusion Arts affecting the enemy rely on the user's soul and spirit.
Normally, these spirits are featureless like water or fog and can't be used as an offensive tool. But illusionists can change their shape, transforming it through their illusion power, endowed with their imagination and will, so as to achieve the purpose of killing people with illusions.
For instance, Bai Kui's Phantom Kui ability, achieved through years of deep meditation, constructs concrete illusion structures. Thereby, it can directly take effect in a battle, creating a realistic environment.
Also known as the Illusion Space.
