The Netherworld is a collection of planes inhabited exclusively by demons and spirits. Rather than being grouped in any particular part of Reshomme, instead the Netherworld exists as a kind of dark mirror to the living planes of the centre and fringes - with each plane separated from it's dark twin only by the Astral Plane. Whether the Netherworld also mirrors the Heavens is unknown, since entry to and exit from the homes of the Gods is closely guarded.
The majority of the Netherworld is made up of spirit-planes - planes that have no physical aspect, but are simply extensions of the Astral Plane. Here most spirits exist without form or purpose, drifting in a dream-like state and waiting to be born - or reborn. This is where all spirits, all souls, first originate - only when a compatible body is born in the living world are they drawn away from this grey half-life and allowed access to the physical world, becoming the soul of the body that called them.
Although the surface levels of the Netherworld are distinct planes, the distinctions between one plane and the next becomes more blurred as you travel further from the living world. In the deepest depths it is believed that all planes become one. This deepest level, the core of the Netherworld, is a mystery to living sages, but it is believed that spirits are first created here. The core calls to all spirits that are not bound to a physical form, summoning them to the Netherworld and binding them there. In essence it is this call that draws away the spirit when a living thing dies.
Nevertheless it is possible for spirits without a body to avoid this trap in several ways; most commonly by pledging devotion to a God. When the devotee of a God dies, the call of the Netherworld is overcome by the more specific summons of their patron's Heaven - and their spirit is drawn there instead. Alternatively some spirits have enough strength of will - or sheer desperation - to linger on as Ghosts or even Angels - binding themselves to some treasured memory of their life, or crafting themselves a new body from raw magic respectively.
Travel to and from the Netherworld is theoretically simple, but in practice it is fraught with danger. Physical travel there is next to impossible (since the rare pockets that have a physical aspect are buried deep and difficult to find), and although a spirit can find its way there with ease it is often difficult to leave again, since the call of the core is so strong. Likewise it is possible to summon forth spirits from the Netherworld but the spirits most likely to respond to that call are not the sleepy spirits of the mundane dead or the shapeless spirits of the unborn but rather those spirits who remember life keenly and would have it any price - in other words: Demons.