What was this place?

It was the Underworld, suppressed by Shoraionji Temple.

What did the trialists come here for?

They overcame the trials in the Underworld to enhance their root bones. Then, they would find the Pool of Afterlife to leave the Underworld and reform their flesh.

This, by itself, was a process.

However, a risk existed: the Pool of Afterlife was not something that one could have if he wished for it. On the contrary, it was the most challenging section in the entire process.

S