Death's Reverie was what appeared in a person's mind when they were about to die.
Tanjiro had triggered it several times in the story, and even Nezuko had experienced it.
Yuko had also gone through something similar once. At the time, she had even complained, Was this some kind of Demon Slayer tradition?
How was it that even she could trigger Death's Reverie?
Because the situation had been dangerous enough, and it had also been when she awakened her mark, Yuko remembered it fairly clearly.
Although she had seen the two older brothers back then, she later learned from Yoriichi that he actually had no idea what she was good at.
So Death's Reverie was merely an image from a person's subconscious.
But that raised another problem.
In the world of Demon Slayer, ghosts genuinely existed.
The original plot had relied on someone triggering Death's Reverie, powering up, and turning the tables. Now that she had saved that person, who knew what would happen if he failed to trigger it?
To be safe, Yuko immediately began teaching Tanjiro Constant Reduplication and training in Sun Breathing, preventing a situation where he could not activate his cheat.
However, Yuko was not in a hurry to force his progress. Constant Reduplication was not something that could be mastered overnight; it required a certain amount of accumulation.
Maintaining Constant Reduplication even while sleeping alone was not something that could be mastered in a day or two.
"Changing your breathing habits right after mastering Water Breathing is difficult, but the Dance of the Fire God is not unfamiliar to you. I believe this transition will go smoothly."
Tanjiro nodded solemnly. "I'll work hard!"
A strange breathing sound rose from his mouth, and Tanjiro's face tightened with discomfort.
Simply maintaining the state of Total Concentration Breathing