"So are you saying the Underworld Soul River was created by someone, and that person was one of my 13 incarnations?"
[Yes and no. Yes, in the sense that the skill was created by your incarnation. However, no, since he wasn't the one who created the Underworld River.]
"I need more context, Tower."
[There are many forbidden things in the universe. Those things, as far as the mind could go, are only meant to be used by the Daos to restore balance to the universe.
They are never meant to be possessed by ordinary men. Of course, it wasn't that ordinary humans can't use it; it was just that nobody is crazy enough to dare take what belonged to the Dao.
The river you glimpsed is the Underworld River. It is the river that takes all dead souls to the underworld.
This river takes the resentment from the souls it carries and sends their free souls to the underworld, making it the most dangerous place in the underworld, just like the Damned Hell River.
