Nine Serenities Underworld, the ultimate yin place on earth, serves as the dwelling for the souls of the deceased.
In ancient times, the Nine Serenities Underworld once erupted due to an overflow of yin energy, causing ghosts to roam across Divine Land, resulting in heavy casualties for the Human Race.
Later, Shennong cast the Nine Cauldrons, placing them in each of the Nine Provinces, suppressing the Earth Vein and thus sealing the Nine Serenities Underworld underground, preventing it from erupting at will—of course, there were still some minor Earth Vein tributaries, exposed in many unknown dark corners, leaking yin energy, which was ultimately unavoidable.
This underground space is evidently one of those smaller tributaries of the underworld. Mortals, with a brief stay, would at best fall gravely ill, or at worst vomit blood on the spot, have yin evil enter their bodies, and die suddenly.
