Death's Master — Tanith Lee

In those days the world was flat and demons dwelled beneath who walked among the cities and kingdoms of the surface with powers and mischiefs to please themselves.

Among those demons there were two who were mighty above all others. One was Azhrarn, Night's Master, and the other was the lord of darkness whose name was Uhlume, Death's Master.

This is Tanith Lee's epic fantasy novel of the strangest exploit of these two demon-lords among the men and women of Earth. It is a novel of odd erotic desires, of twisted ambitions, and superhuman feats. It is the story of two boys who became men under the stresses of witcheries and wonders that surpass even the fabled lore of the Arabian Nights...and the story also of queens and witches, of kings and commoners -- and of the two terrible lords of darkness.