As Tetsu no Sosuke bandaged him, Tanjiro suddenly remembered something. "Sosuke, Nezuko's missing. I have to find her."
"Nezuko isn't the only one who's disappeared. The others are gone too." Once Tetsu no Sosuke finished bandaging him, he drew Tanjiro's blade. "Watch closely."
He cut his own hand, and blood trickled down.
Tanjiro stared at him in utter bewilderment. "What are you doing?"
Was this supposed to be an apology?
"This is the Spiritual World. Everything we see here might not be real," Tetsu no Sosuke said. "Our swords are Nichirin Blades. A cut alone might not prove anything, but a demon would suffer far more pain than we do."
"..."
Only then did Tanjiro understand what Tetsu no Sosuke meant.
He rolled over and sat up. "I was with Nezuko earlier, but she was a fake."
"How did you figure it out?"
Tanjiro recounted what had happened. Earlier, deep inside that room, he had suddenly realized that if it really was a room—an enclosed space—he might be able to smell a change in the scent.
After all, the scent was faint, so faint that even Tanjiro's nose could barely distinguish it.
Out in the wilderness, however, even Tanjiro's nose would have struggled to notice anything wrong.
In fact, they had been out in the wilderness at the time, so the appearance of that scent might have been another layer of the scheme.
Because of that, he had kept a close eye on the sister who had suddenly appeared beside him. He eventually noticed something suspicious, but her surprise attack still caught him. She had kept him on the defensive almost the entire time, until something unexpected apparently happened and he was knocked right in front of Tetsu no Sosuke.
The "Nezuko" who had been beating him up all along