Paradise Island, the Waterfall of Truth.
It was a magical waterfall. Simply sitting before it allowed a person to see the unknown self hidden deep within their heart.
In his previous life, Naruto had fought his inner darkness here.
It was said that Tailed Beasts were incarnations of evil and resentment. If someone harbored even a trace of darkness, they would immediately lose themselves upon using tailed beast chakra!
Thus, training at the Waterfall of Truth was also a necessary step toward becoming a perfect jinchuriki.
Later, as his horizons broadened, Naruto learned more of the truth.
Tailed Beasts were not pure incarnations of evil as the rumors claimed. Becoming a perfect jinchuriki fundamentally depended on reaching an understanding with the Tailed Beast.
If that could be achieved, even someone with evil in their heart could use a Tailed Beast's power, provided the Tailed Beast did not mind.
But Tailed Beasts generally had good hearts, so eliminating darkness did help.
Another truth concerned the Waterfall of Truth's "true nature."
"Like Hell Valley, this place is one of the legacies of Six Paths Ninshu. The waterfall contains powerful Yin Release energy."
With sufficient spiritual strength, a person could see not only their inner darkness, but even other beings!
In his previous life, Naruto's son, Boruto, had also met Momoshiki at the Waterfall of Truth to gain control over the power called Kāma! [A setting detail added by the author.]
During such encounters, the waterfall created an illusory battlefield visible only to those involved. Nothing that happened within it affected the outside world!
Now, Naruto sat on the stone platform before the Waterfall of Truth and calmed his mind!
Whoooosh...!
The naive Naruto of this era walked out of the waterfall and sneered, "Yo, you impostor occupying my body! You finally worked