Regarding their missing souls, the deductions had never felt anything obvious, but they had actually noticed it in certain details.
For example, they were more easily affected by ghosts and monsters, and their ability to resist them was far weaker than before. They would also occasionally feel dazed and lose the ability to move when they saw a ghost.
They had already suspected as much. The "missing souls" mentioned in the Main Quest might be a side effect of their deaths in the opera-stage world. Their souls' instability could be what made them more "afraid" of ghosts drawing near.
Now, after hearing what River Witch had said, their suspicions had been confirmed.
And the news that followed was simply the worst possible kind.
—The Forbidden Art of Living Souls had an even greater restraining effect on people like them, whose souls were unstable.
In other words, even if their willpower and abilities had once been enough to resist Master of Ten Thousand using the Forbidden Art of Living Souls right in front of them, they couldn't do it now. Their incomplete souls and bodies were the perfect stage for the art. Perhaps with just one encounter, Master of Ten Thousand could pull their souls out again.
It wouldn't matter if he didn't put them into a new opera-stage world. As long as their souls left their bodies, Master of Ten Thousand could easily seize the opportunity to destroy their bodies, forcing their souls to become undead.
Their only remaining chances were either to subdue Master of Ten Thousand before he could use the Forbidden Art of Living Souls, or to run back the instant their souls left their bodies and return to them before their bodies were destroyed.
Either way, they would have very little time to react. It might