On Narth, once called the “continent of man”, the origins of the Demons are obscure. Some say a wizard’s summoning went out of control. Others suggest that the Demons themselves opened a magical gateway, still extent at the gate of Tor’zem, the Demonlord Capital. A few philosophers even believe that the Demons are a natural race, like humans or dwarves, except the Demons became stronger than others.
In any event, the Demons are now the power on Narth, and each Demonlord rules his own province under the Emperor. Although Demons are a tiny minority, through their great power, magic, and capacity for evil they act as captains, administrators, and governors of many lesser races such as half-men, demi-men, goblins, orcs, and other manish races of darkness.
Nisshar is a typical province in the west of the Demon domains, ruled from a capital city of the same name. But here, humans and semi-human allies resist them, banding together under the influence of Hosar, a sun-god cult. At times, they were so successful that even Nisshar came under siege. Now, the armies of the Demonlord and the Alliance of Hosar are mustering once more, for another bloody campaign.
After centuries of sleep, the Demon Empire once more sends forth its hideous armies to conquer the lands of men. Desperate fighters stand with bow and shield against the goblin hordes which blacken the fertile valleys. The wizards and priests are called together in a desperate attempt to counter the Demonlord's dark magicks. But the balance of power lies with the neutral kingdoms. Will the Dwarven King, the Cloud Prince of Lyung, and the mysterious Ancients join with the forces of light, or will they cast their lots with the dark host commanded by the shadowy presence of the DEMONLORD...
The second idea is a riff off the first, a post by Philotomy on Original D&D Discussion and the really neat idea for Project Long Stair from RPGnet. It would be set after the first idea,
Many of the Mages of Hosar retreated to their tower in the great city of Timur. It is now said that they went knowing full well their upcoming fate. These great mages wielded arcane power beyond any seen in any age since and with this vast power they were able to hurl the Demons back into the primordial chaos from which they came.
However, a devastating backlash of arcane power and the forces of chaos destroyed Timur and everything for leagues around. The Mages of Hosar were devastated. All of those that participated in the Great Sealing were lost. Only the few that were purposely kept away from the ritual survived. While the price was high, the Great Sealing forced the Demons back into the chaos and sealed the gateway to Narth. That was centuries ago. Now the seal is failing...
The idea is that the seal and gateway are part of a megadungeon that is a mythic underworld. This mythic underworld is spreading tendrils, magical portals, into other places of darkness and chaos (smaller dungeons and lairs).