The Underworld was formed with the deaths of several of the Primordials during the Primordial War. Since such massive and powerful creatures had never died before, in a sense they refused to leave Creation as such, the Underworld was created as a dark mirror of Creation. The dead Primordials themselves came to be called Malfeans or Neverborn.
The geography of the Underworld is very similar to that of Creation. The main difference is the lack of elemental poles in the Underworld. Where lies the Blessed Isle in Creation, the Underworld possess its own Isle of Sorrows, and the city of Stygia - which otherwise has no counterpart in Creation - occupies the same place as the Elemental Pole of Earth. Notably, where Creation has an immensely tall mountain, Stygia has a large pit leading down to the Labyrinth and, eventually, Oblivion.
In certain places, the boundary between the Underworld and Creation is weak: such areas are known as Shadowlands. Anyone entering a Shadowland enters a twilight half-world. Both ghosts and the living can inhabit it, although not at optimal conditions for either side, as it is a combination of both realms. When departing a Shadowland, one will arrive in Creation during the day and in the Underworld at night.