Amon's take on what was happening was that Yon Yu was learning how to use her unique power. She was synchronizing with the Pledge Demon at a rapid rate, as evidenced by the Pledge Demon illusions that were chasing them. These illusions were slow at the start but became faster as time passed.
Amon and the others could easily dodge these illusions' assaults. Xian Yang lost one arm because of his conceit. If he actively evaded the illusion, he wouldn't have been wounded. However, that was before. Currently, they were using their movement arts non-stop, but they were starting to have trouble evading the illusions. These illusions were catching up to them.
At this rate, Yon Yu could gain more power. She might control everything within her domain. Even if they chose to flee, they wouldn't exit this village quickly enough before Yon Yu reached that pinnacle of power. She could trap them in an even stronger illusion and prevent them from fleeing.
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