At the dawn of time the gods who inhabited the Astral Sea warred with the Old Ones from beyond the known universe. Aberrant monsters of the Far Realm they sought to unmake creation. The gods were hard pressed and imbued the mortals with their powers to join in the battle. In the end the Old Ones were driven from the moral world and it’s parallels as well as the Astral Sea. However, many gods fell and a handful of openings to the Far Realms permitted the servants of the Old Ones to corrupt creation.
For a time the gods worked to heal the world and mortals hunted down the fell servants but the Old Ones were not permanently deterred. Again the Old Ones struck and were defeated yet more gods fell. Like the tides breaking upon the shore each war ended with reality preserved but more of the gods, and often their planes within the Astral Sea, destroyed. Each time more of the Old One’s servants remained behind.
Now, a thousand years after the last war, the world awaits its fate. The Last God imbues his servants with the power to destroy the aberrant monsters while runepriests join mystery cults dedicated to fallen gods to learn their powers. Other mortals, knowing that the gods are spent and the defense of the world falls to them, have taken to walking arcane paths and making fell pacts to garner power to defeat the Old Ones and their servants. The Fey wilds have sent forth their own champions and even beings of Elemental Chaos have stepped into the world to prepare for its next defense.
Yet even before that can begin the world must be cleansed of the corruption left by the Old Ones, including the promises of power to persevere against them or even among them. It seems no place is pure and no power lies uncorrupted.
This idea comes from two primary sources and one secondary source. The primary sources are James Raggi’s how to make D&D metal and Palladium Book’s Old Ones. The secondary source is Charnel Gods, a supplement to Sorcerer which has no web presence that I can find.
The principle idea behind the setting is an ongoing war for the universe between the gods and Cthuvian Old Ones. As the gods beat off each attack their numbers diminished faster than their ability to regenerate. Now, the Last God prepares for his final battle by empowering servants, while men try to claim both the powers of the fallen gods as well learn the powers of creation directly.
Meanwhile, these wars have corrupted the world itself. Most power is now seduced by servants of the Old Ones and the world wars among itself as much as it tries to heal and prepare. The characters are new heroes rising to fight the corruption of the world only to risk seduction by it. Those who persevere beyond that seduction can rise to defend creation itself.
I originally conceived this for my restricted classes and races 4e campaign.