The lands of Sheol were once adorned in immense and beautiful forests, vast colorful canyons, and societies of mortals advancing: this changed in, seemingly, the blink of an eye after the land was quickly taken over by fiends at the same time the outskirts of the land were coated in burning mist. Parted in many areas by an extension of the River Styx, Sheol is constantly at war with its two residing sides of fiends: the demons and the devils.
The regions controlled by the devils are more regimented and built into functional cities with strict, extremely binding laws. On the contrast, the demons reside in the spaces between these devil-controlled cities, whether mindlessly stampeding through wastelands, gathering in tribes, or advancing in what a demon lord considers to be a "city".
It is not known which of the two sides made the first move on the material plane, but each side accuses the other. According to the demons, the devils were trying to reach beyond their means and claim the material plane as a slave order in their military regimes and, as some intelligent demon lords would call it, "manipulative bureaucratic devilry". According to the devils, the demons quietly expanded into the material plane and used their demonic twisting presence to usher along portals connecting the Abyss and the Material Plane, and the devils were forced to follow in order to stop them expanding further.
The mortals who are native to the plane are divided in some parts but extremely united in others. Some mortals saw resistance as futile and joined the legions of the devils, some were destroyed or even corrupted by the demons, but many others chose to unite against these invading forces and created a few cities that stand against the fiends, hoping to banish them back to whence they came. Though, over time, many fiends intermingled with various races of mortals and created hybrids in quantities not seen before on other worlds. Thus, the lines of loyalty get blurred in some areas of Sheol, and there are a couple communities who live in harmony during this dark time.