Nightmare
Shen Yu was really surprised by what he was seeing before him. If viewed by mundane eyes, there was nothing. But if someone tried to sense, they would have their mind collapse or at least be damaged depending on their cultivation base.
It was because a tiny translucent being was trapped in mid-air, experiencing thousands of nightmares woven carefully by Vanish with her illusion technique.
It was frightening to say the least, and what impressed Shen Yu the most was how many layers she was able to stack on the initial illusion to make it look like an endless nightmare.
Even if someone broke through one nightmare, they would experience another, and this cycle would just keep repeating until either all layers were destroyed or the trapped person lost themselves in those nightmares.
A true horror was right before him.