Nekroheim was the first underworld from which all other underworlds had emerged. 
The vast expanse was lit by a dead sun, an ancient orb long since snuffed is light stretched upwards from the deepest part of the abyss. The dead of entire civilisations had been used to form the walls and ceiling with a sickly radiance casting a shrouds of frost and fog.
In the Time of Tribulations Nagash finished the construction of a new vast Black Pyramid that stretched down into its depths, its base stretching across the city of Nagashizzar above. He enacted his Great Ritual and a vast storm of power broke through the borders that separated Nekroheim from the other underworlds to draw them towards Nekroheim, becoming part of the growing maelstrom. Nekroheim crumbled and changed shape, bending beneath the Pyramid, bowing up around it.