In the book
The Underworld is a private to souls, or Shades (as they are confused in the books), of Fillory. It's a gigantic gymnasium with brick walls, where all the souls play games to distract themselves of the boredom of death. There are ping pong tables and rackets, volleyball court, etc.
It's accessible to the living through a ritual that creates a steep, winding slide that ends at a pair of closed doors surrounded by darkness and illuminated by a single overhead light. The doors themselves are guarded by a spirit resembling a kid, and a passage is granted to any who possess a 'passport', or any other identifying documents. Quentin's passport from Outer Island was an acceptable document.
In the TV show
It's normally only accessible to souls, and is a complex system of smaller ecosystems that are designed to make people feel comfortable enough to "move on". It's not entirely explained what "moving on" but it's implied that there is Hell, mentioned in a brochure, and something better.
If someone desires to enter the Underworld and come back alive, they must seek the help of a Gatekeeper, and let their bodies rest in Earth, while their souls travel through there.
The Underworld has its monarchs: the gods Persephone, also known as Our Lady Underground, and Lord Hades. They made all the rules that exist down there, but both are absent in present day. One apparent rule is that no one responsible for someone's death is allowed to be with the victim. There's a strict policy about processing Shadeless people, but it's not explained what happens to them.
Structure of the Underworld
The Underworld is broken up into many provinces, or ecosystems, with each serving a purpose regarding the souls that inhabit the dimension.
The Underworld Meadows
The Meadows is a place of transition that resembles a modern waiting room where souls staying while waiting to be sorted out to their fate, according to their karmic cycle. This place is managed by Underworld officials that deal with dispatching the souls, account for their transitions in the Hereafter, and archive the names and backgrounds of the deceased.
Elysium is a sumptuous, richly decorated place that resembles a real estate. This place is home to the souls and Shades of the dead, and is likely to be a place where Our Lady Underground used to reside, as one of her portraits is affixed to one of the walls and one of the souls said to Julia and Quentin that 'Ms. Persephone' usually spent time at the Elysium before her disappearance.
Although not shown, the existence of Hell is attested by the brochure read by Quentin in the Underworld Meadows. When Marina Andrieski was briefly resurrected by Julia and Kady she was afraid to go back into a dark place, possibly confirming her soul was confined in Hell. It can be inferred that Hell serves as a prison for dark souls, similar to "Tartarus" in mythology.
- Julia and her Shade (formerly)
- Alice's Shade (formerly)
- Quentin (briefly)
- Persephone (formerly)
- Martin Chatwin's Shade (presumably)
- Benedict Pickwick