The Netherworld possessed the "Constant Existence" effect and could capture enemies within it, inherently equipped with sealing and subduing capabilities.
The Wraith Sacred Mountain still lacked the ability to seal, and its subduing ability only manifested when using the mountain as a Great Seal to suppress the area.
The Underworld Ghost Market did have the capability to trap external enemies, but doing so would continuously drain the energy of Hong Yi and the Ghost Market.
The sealing and subduing powers of the Netherworld, even the concept of Constant Existence, were naturally existing.
Enhancing these forces, or activating the effects of the Nine-layered Netherworld Prison might require energy, but simultaneously, the Netherworld could devour evil deeds and form a Power of Sin several grades higher than the energy of heaven and earth.
In some ways, imprisoning felons in the Nine-layered Netherworld could achieve a self-sufficient perpetual motion effect.