Setting
The Underworld shouldn’t exist. When the Primordials made Creation, they never intended for anything to linger after death. Their system was flawless: something died, it went through Lethe, and it reincarnated. There was no waste, no hemorrhaging of souls, nothing. Of course, their system was never meant to accommodate the Primordials themselves.
When the first Primordial died, it found it was too huge a concept to be digested by Lethe. Metaphysically, it was the equivalent of a clogged drain. Its weight carried it… somewhere else, where it lingered, mostly asleep, dreaming of vengeance and destruction. Slowly, as more and more Primordials died, their dreams added together and became a pale reflection of their Creation. Furthermore, mortals found that they did not have to pass through Lethe, but could linger as ghosts in this Underworld.
But the fall of the Primordials left a crater, burrowed a hole into nonexistence, called Oblivion. This hole exists in the mirror analogue to the Imperial Mountain, a well in the center of the Underworld that stretches downward forever, until it culminates in absolute Nothing. The slumbering Neverborn wish to pitch all Creation into this Oblivion, that they might finally shed the shackles of their ties to Creation and cease their eternal tormented nightmares.
Geographically, the Underworld is the mirror of Creation, save that it seems… well, dead. The sun is less bright, the nights are darker, and everything within seems listless and pale. Every kingdom that ever existed in Creation has (or had) a shadow in the Underworld. Some of them thrive now more than they ever did in life, while others find they have fallen before the kingdoms who followed them. Ghosts from every era can be found in the dark corners of the Underworld, some fresh and new, and some terrifyingly old and insane.
Worst of all, some places are simultaneously Creation and the Underworld, joining the two like a single patch sewn onto two sheets of fabric. These shadowlands allow the Neverborn and their spectral (or Abyssal) minions to travel between the worlds, giving them a foothold in the land of the living.
With the return of the Solar Exalted and the emergence of the Abyssal Exalted, the Underworld may yet be the final battleground for the fate of Creation. Mastery of Necromancy might yet provide a solution for the slumbering Neverborn, while nihilistic Abyssals might yet plunge all that exists into Oblivion.