For many thousands of years man has wondered what lies beyond the known world. Through the sands of time many discoveries were made that led to the Advancement of human civilization culminating in the conclusion to the first question when contact was made with the Sentient Beings that have come to be known as Nomads, til this day man fights them in a struggle to survive! So it would seem therefore that there is nothing more to the Physical World than what is already in it, But all the this time man has overlooked the very realm in which reality is entwined with. The Underworld a realm spoken only in myth and legend where the souls of the dead go, when their mortal bodies no longer cease to function has always been existed.
THE BLACKSCIENCE:
In the 20th Century scientists began to detect the gravitational effects of darkmatter upon the physical world via the unexplained rotational speeds of galaxies, where upto 70% of the mass of such galaxies was undetectable. Little did they know of the world that darkmatter makes up nor what goes on within it!
The Underworld is unlike anything known in reality, the physical world cannot interact with it cannot comprehend for the very matter that makes up the Underworld World and Everything in it is undetectable, except for the Gravitational Effects this matter has upon the Physical Realm it surrounds. Dark Matter is what the Underworld is made up of, and thus any Undead Spirit that is also a Dark Matter form will pass straight through any object of the Physical Realm. It is for this reason why the underworld has remained only in legend, humanity cannot envisage this alternate dimension. Dark Matter has another more sinister property which Dark Matter forms can exploit over the physical realm, through an UNKNOWN MECHANISM are able to change the nature of the dark matter that they are made up of in order to interact with the physical world. Thus darkmatter forms (Souls) are able to pass between the Underworld and Physical Realms effortlessly. This would explain how throughout history darkmatter forms (souls) of the undead realm have been able to taunt and harrass beings of the physical world in one instant and then disappear the next.
DARKMATTER REALM:
The underworld is a unforgiving place for the dead, usually when a soul passes from one realm to another it is frantically disarrayed by the horror it faces. Souls are punished for eternity for the life of bad they led in the physical realm, Angry with the punishment befallen to them over time these Trapped souls become Blackened with hatred for the decadence of the Physical Realm. These souls are known as Demons, and become the Henchman of the Guardian of souls in the Underworld.
Known only the as the Boatman it is his Job to the ferry of Souls (Darkmatter forms) from one realm to the next. The Boatman is Overload to all that dwells in the Darkmatter realm "Underworld", his origins are not known but what is known is that he is evil and will stop at nothing to maintain equilibrium in the underworld realm, this balance is only truely maintained by the constant in flux of dark matter into the Underworld. As the very nature of darkmatter itself is so volatile that without a critical mass of darkmatter, any dimension it makes up would collapse in on itself. Thus the boatman requires as many souls as he can muster, for souls are made up of the very darkmatter that is required to maintain equilibrium!