

UNDERWORLD:
Laying beneath the Material Plane, the Underworld is a domain of ghosts and memories, a gloomy and faded reflection of the land of the living. The dimension is inhabited both by the ghosts with unfinished business and couldn't go to Heaven nor Hell, for a reason of another; and by stranger spectres of unknown sources. It is not a safe place for the living to venture, for some ghosts are sorrowful, some are vengeful, and some are mad. The Underworld is bleak, but often punctuated with bright colors, reminders for the dead of the living world. This place offers many dangers but many rewards.
Access :
The Underworld can be entered through an Avernian Gate, a portal to the upper levels of the Underworld. Avernian Gates occur in places associated with death or that had been saturated with it, like graveyards, morgues, mausoleums and battlefields. Avernian Gates need a special "key" to open them, a ritual of varying complexity, but supernatural individuals can open them without using a key.
The Bound can use their Geist to open the Gate, while mages can use Death magic for it. Some vampires can open them with Blood Sorcery, while some Fair Folk can use magic that deals with death and the underworld. Powerful angels and demons can simply force a Gate open.
Geography :
The Underworld is divided into two main layers, the Autocthonous Depths and the Lower Mysteries, or Underworld proper.
The Autocthonous Depths are in fact an intermediary realm between the Underworld and Earth, similarly to Hedge that separates the Material Plane from Faerie. The Depths take the form of a vast system of mazelike tunnels and caverns, and reflect the culture and geography from where they were entered, appearing as subways, catacombs or simple caverns. Most of the ghosts in there are young, having died no more than a century ago, but it is not impossible to find older ghosts and Geists in it. This is where most visitors come to do their business.
After crossing the labyrintical Depths, one reaches the Lower Mysteries, the true Underworld. Unlike the tight and claustrophobic tunnels of the Autocthonous Depths, the Underworld is vast, open and filled with many different landscapes, all appearing dead in some way. The light coming from the sky is pale, and everything looks grey and faded. There are vast plains covered in grey dust and ghostly grass, forests of dead trees with no leaves, great mountains of exposed jagged rock and rivers of foggy water with strange and mystical properties. Veins of gold, silver or platinum and clusters of gemstones are very common, often bursting from the ground, and thus are considered almost worthless by the dead.
As mankind developed civilization on Earth, so did their dead in the Underworld. As they build cities, temples and palaces, reflection of those started to appear in the Underworld, and the ghosts that inhabited it began to organize in search of protection from the darker beings that were there before them. This led to the creation of multiple nations of the dead, ranging from small city states to vast bureaucratic empires.
Notewhorthy locations include, among others, the Ocean of Fragments, the Athaeneum, and the Agorean Bazaar. The Ocean is an incredibly deep sea of black made from fragments of forgotten memories. Submerging into it will slowly erode one's memories, and it's said that if they swim to the bottom, they will cease to exist. The Athaeneum is a gigantic library, a reflection of the glory of of the Library of Alexandria. In it, all knowledge of mankind, even if forgotten or destroyed is stored, but finding a specifc book or scroll is almost impossible, due to the sheer size of the library and to the tireless sphinixes that guard it. The Agorean Bazaar is a vast, labyrinthic marketplace where one can buy almost anything, from soulforged weapons to Mementos, reflections of items connected to major deaths, giving them unique powers.
While the Underworld lacks true weather, being a dead and still place, it does have form of natural event, the Tempest. Usually the "sky" of the Lower Depths is calm, covered with a thin layer of grey clouds, but when great massacres and disasters take place, causing many to die suddenly and at once, it grows restless, and a Tempest is formed. The Tempest is a massive storm of destruction, composed not only of water and wind like an earthly storm, but also of broken glass, acid, fire, burning Plasm and many other horrific substances. Also, Tempests happen much closer to the surface than regular storms, and flow not only downwards, but also upwards and to the sides. Luckly, it takes incredible amounts of death for a Tempest to form, making them not very common.
The only substances considered truly valuable are Plasm and Stygian Iron. Plasm is a metaphysical and undefined "stuff", composing the bodies of ghosts and other inhabitants of the Underworld. Most Plasm exists in form of energy, permeating the Underworld in a form similar to mana that some beings can manipulate in order to alter their surroundings, similarly to Mages and Fae. Truly workable, "physical" Plasm, on the other side, is extremely rare, only appearing in liquid form or as solid shards after Tempests, or rarealy as veins of solid plasm under the surface. Plasm is used for Soulforging, turning it into material for crafting tools and artifacts used by Underworld society. Soulsteel weapons are some of the few that can hurt ghosts. Stygian Iron is even rarer, only extraced from a single mine known as the Veinous Stair. It is a dark metal, lined with flesh colored veins that seem to subtly glow and pulse. Artifacts made from it are even more durable and strong than Soulforged ones, and only the highest ranking members of Underworld society have access to it.
Inhabitants :
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Ghosts
The majority of the inhabitants of the Underworld are Ghosts, remnants of souls who departed from the world of the living and went on to the next world. They still hang on due to unfinished business, uncapable of moving on.
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Geists
Some ghosts, after spending enough time in the Underworld, become Geists. Geists are stranger beings, ghosts reshaped by the Underworld into powerful representations of death with a craving for influencing the living world and feel life again. This allows them to make pacts with humans who had just died or passed through near dear experiences and bring them back to life, in exchange for sharing their flesh and going to the world of the living with them. Those human-Geist hybrids are known as the Bound.
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Bound
The Bound, also called Sin-Eaters, are humans who had experienced sudden or early deaths and have been given a second chance life by making a pact with a special ghosts known as Geists. Many humans who become Bound were already closer to the world of the dead and the supernatural, but others were simply humans with enough will to live to accept the pact and come back.
The relation between the human and Geist may vary depending on how friendly the Geist is. Some Bound see their Geist as a friend and ally, while others are tortured by the voice constantly whispering inside their mind. Still, for those lucky enough to have a Geist that likes them, their new situation has far more positives than negatives. Not only do they get a new chance in life, but they also recieve many new abilities. They can see and communicate with the dead; can use their Geist for a variety of ghostly powers, known as Manifestations; and can use Plasm to heal themselves, open Avernian Gates and power Ceremonies, rituals of necrotic magic more complex in execution and result than Manifestations.
How they use their new powers depend on the Bound, but coming back from the dead makes most of them fairly easy-going, so few Bound are actively malicious. Many of them use their powers to help other ghosts move on to the afterlife or to deal with those that refuse to do it, and have founded societies dedicated to such objectives, the oldest and best known of them being the Oath-Circle of the Oar, also called the Boatmen's Society or the Ferrymen.