Olympus by Night
The underworld is an alternate plane, a shadow realm that grew around the Void Prison which holds the Titans and Primordials back from Creation. There are 3 layers in the underworld.
The first is the Underworld proper, and a majority of ghosts live here. The city of Stygia, the rivers Acheron, Styx, and the Dead Sea, the pyramid cities of the south, the wintery shores of the North, the marshes and fields of the east… When most people think of the Underworld, it is this layer they think of.
The 2nd layer what Christians call Hell. Here is where the wicked go to suffer, and terrible gods and goddesses live to punish them. These realms are cut off from one another, but can be accessed from the Underworld proper, and occasionally these realms war with one another or spill into the main Underworld.
The 3rd Layer is the Void Prison and the Well of Oblivion.