After the Ghost Realm Descended, I Built the Underworld on Earth

Rebuilding Heaven or the Underworld with a System

The protagonist transmigrates to a world with neither a Heavenly Court nor an underworld. They get a System and gradually establish one of the two, eventually becoming its ruler.

Any Stories Where the Faction the Protagonist Creates Is Called the Underworld?

...I remember reading a book a while ago where the protagonist joined the mysterious, high-powered forces of the underworld. That's all I can recall.

I Built My Own Underworld, Complete With Black and White Impermanence