When we drift off to sleep our consciousness dives into Abzu, Baphomet, the Great Sea of Souls at the center of the Earth. It is the sum total of all consciousness on the Earth, a vast pool of all life, woven together in an undulating mass where the borders of the ego dissolve and the self is slowly lost in the vastness of No Self. This is the Dreamworld, the underworld, the land of the dead. When one dreams one sets sail on this great sea, drifting through a collective landscape of thoughts and fears, hopes and desires, interacting with disembodied spirits and the souls of others.
As long as the body still lives the consciousness is anchored to the material plane and can conceivably return to its corporeal chassis without fear of becoming totally lost in the Abzu. In the event of death, however, when the body is damaged or destroyed, the souls connection to this world is severed and there is nothing to actually return to. Therefore the act of dying is essentially the same as the act of going to sleep, the only difference is that in the case of death, there is no returning, not in the current form anyway.
A living, conscious being is made up of three parts; the Spirit, the Soul, and the Body. The concept of the Body is easy enough to understand, it is the material form that exists and can most easily be perceived, you can see it, touch it, etc. The Soul is essentially psychic baggage, it is the acquired sense of self, the feelings and memories and tastes and general personality of someone. It is the ego, it is the thing that identifies itself by a name. The Spirit is the eternal spark of a being, it is the True Will, it is an actual fragment of Divinity, the God that lives within us all. To make an analogy; The Body is like a taxi cab, a physical mode of conveyance; the Soul is the driver, piloting the vehicle and deciding the actual course; and the Spirit is the fare, the one who decides what the ultimate destination is, the reason the cab is doing anything at all.
When one dies, the body then begins to immediately break down. The Spirit moves on, continuing on its course onwards and upwards to infinity, Excelsior. It will take on other forms and other incarnations in its ongoing Quest for completion. The Soul begins to fragment, part of it stays with the body and dissolves with it, part of it may linger on the material plane, a shadow of a memory, haunting the places it used to know and dwelling in the minds of those who once knew it. With time it too will fade. Most of the soul will cling to the Spirit, at least at first, as the Spirit moves on. Part of the soul is absorbed into the fabric of the Spirit itself but the bulk will dissipate as the Spirit drifts upon the waves of Abzu, the shell of its former self slowly pealing away. Sometimes the soul does not want to let go and the process of doing so can be difficult, even painful. The Soul may put itself through its own personal Hell as it dwells by itself in the Dreamworld, clinging to an evaporating past. Eventually when it learns to let go it can then move on, rejoining the rest of consciousness, the combined memories of All. The Spirits journey does not end in the underworld, it does not linger for long. Once it has shed the parts of the self that arent absorbed it is free to continue in its course and do as it will.
The Dreamworld is Sheol, it is Dis. One can wander in its folds for a long time, the longer one does though, the more time will lose all meaning. It is here that we go when we go to sleep, and it is here that we go when we die. It is easy to become completely inundated here in this realm of ethereal distraction, especially without a physical body to provide a grounding barrier, the fuzzy border where the self ends and everything else begins becomes more and more indeterminate.
With practice you can learn to master your dreams, you can learn to become and remain lucid, remembering and controlling all that you see. In dreams you can gain prescience, you can communicate vast distances, you can even effect the waking world. Through learning to master your dreams you can even learn to master Death itself.