It was not an illusion.
In the beginning, Chu Nan thought that Prince Montique's cultivation of the Exterminating Heart Technique was not profound enough, so his performance was not good.
However, after fighting for a long time and coming into contact with Prince Montique many times, Chu Nan discovered that when he used the Exterminating Heart Technique, the surrounding spatial energy was affected by it. The degree of decrease in spatial energy activity was clearly stronger than Princess Romanti's.
According to Chu Nan's understanding of the Exterminating Heart Technique, this clearly meant that Prince Montique's cultivation level in the Exterminating Heart Technique was definitely deeper than Princess Romanti's.
As for the precision of Prince Montique's use of spatial energy that he had first sensed was inferior to Princess Romanti, after a period of battle, it displayed a very obvious difference.