When it happened, it happened simultaneously.
Ground is sanctified for a reason, to take it away from evil. But sometimes evil cannot be removed. It is buried deep in the dark depths of the earth. Sealed by the erection of a sacred building atop their grave. A perverted gravestone to keep the dead in the ground. It was once well known, prayers were said and rituals were carried out to prevent the demons from escaping, but as time passed and demons fled this world, those rituals were changed to myth and legend.
The spells and chants that kept them in their prisons were used less and less, weakening the protective charms. They stored their power, sharing it amongst themselves, something no one could have predicted. Demons are greedy. They build up their power, their fortunes and guard it jealously. Anyone, demon, man, angel, spirit, who comes near it is almost certain to die, whether from the demons attack or a curse they have laid on their bounty.
The world shuddered and convulsed as they broke free, vomiting them from it’s depths as they burst through the buildings designed to imprison them. They were free. But with their freedom came a shocking revelation. They were all that was left. The other demons had long ago departed, driven out by humans. Abandoning them in their prisons. Anger turned to glee as they realised that now they were in control of everything. With much fewer demons there would be less battles for power.
The world was unprepared for their revolution, millions died, torn apart in an orgy of blood and violence. A defence was mounted, but it failed, their weapons having no effect on the demons. They would be wounded, but could heal at fantastic rates. The old myths and legends, long ago discarded and forgotten held the key to how the demons of old were driven from our realm. But now hope was lost. There was nothing to be done.
The demons scattered around the globe, staking out their own claim on lands, ruling them as they saw fit, becoming lords and masters over all they saw. Demons destroyed buildings and land, making nests for themselves. Commanding their slaves to provide them with sustenance along with anything they required.
The humans were to farm the land and raise animals, they were given just enough to survive and anything else was given to the demon to do as he pleased. Humans were allowed to marry and breed, but with strict controls. If too many children were born in one year, an adult was sacrificed to make room. Likewise if someone became to old or sick to work, a child had to be born to replace them.
The demons sat on their thrones and watched as those around them did their bidding. At first, they were satisfied by what they saw, but, as is a demons nature, soon it was not enough. They commanded humans to breed more, forcing the strongest to mate before sending them to battle. Their dread armies marched across the land, killing everyone they came across, battling one another for supremacy, weakening enemy demons until their master could kill it. Empires slowly built up around the demons, each society believing theirs to be the strongest, they fought ferociously against all those who would overthrow their king.
Anyone who was found to have escaped the demons were sacrificed, as were those who were taken prisoner.
As the years passed so to did the demons, the weaker ones failing and falling, as they stronger absorbed their land and people, those unwilling to dedicate their lives to their new masters were executed publicly and their bodies and souls consumed.
For centuries battles raged around the world until there were only two demons left, their armies equally great, both fearing defeat. As their armies marched to war, they burnt the earth behind them, both jealously destroying their land lest it fall into the hands of their enemies. The battles lasted for years, wave after wave of men, women and children fought one another, each dying in their turn. The ground was soaked in blood, bathing the entire globe. They fought until the last fell, leaving no one left. The demons rose from their dens to meet each other in battle. They fought for decades, neither gaining the upper hand.
In their final battle each killed the other, knowing that neither could win, each in turn decided that they would kill the other while they themselves died. When the dust settled and the smoke cleared the planet was barren. The seas had boiled, the sky had been filled with smoke. The rain that fell scorched and scalded the earth.
The great civilisation that was man had ended and any evidence of their presence was burnt from the earth until there was nothing left. In time, small pockets of vegetation that had survived the harsh conditions began to spread out, cleaning the world. Animals began to flourish once more, years went by, then decades. millenniums passed before the world was once more thriving with life, a lush and verdant beacon in the cold harshness of space.
But even as the earth was entering its final stages of recovery, the demons began to return. Dominating the land once more, mating with animals, produce unholy offspring, intelligent and cunning. As they began to plot to overthrow their creators, the never-ending cycle began anew.