Supreme God of The Underworld, Grim Reaper King

The vast universe contained two supreme existential planes—the supreme Celestial Realm and the supreme Netherworld.

The former was owned by the deities, the latter by the demons.

In nature, the supreme Netherworld wasn't different from the supreme Celestial Realm. They had the same laws of nature, space structure, energy constitution… The difference was only that the lifeforms that occupied them were different.

The supreme Netherworld was as vast and boundless as the Celestial Realm. Inside it lived countless dark creatures, such as the Dead Souls, the Necromancers, the Specters, the Undying, the Abyss Demons, the Fallen Demons, the Skeletons, the Purgatory Kings…

These dark creatures were extremely powerful. They didn't cultivate a divine body nor the laws of nature. They grew simply by devouring their own kind. As a result, there weren't many who could really grow to the pinnacle.