Exiled Future is the name I've given to a campaign world flavored post-apocalyptic fantasy. Well, not exactly. The world is post-post-post-post-apocalyptic fantasy. Inspired in large by the original Dying Earth stories by Jack Vance.
Picture a world where a single empire has conquered everywhere. Their reach is unchallenged from shore to shore, ocean to ocean, perhaps even to the moon and sun. Where do they put the prisoners, the dissidents, the rebels, and those who simply don't fit into the grand planned order? Caverns and tunnels deep in the world are found with magic. So far that there are no exits or entrances. Only the most powerful teleport rituals can reach there. A one-way trip. So all the undesirables. The criminals, the political prisoners, anyone too powerful to handle, they all get sent down. Exiled to the caverns large enough to hold entire kingdoms.
Eventually the empire collapses, centuries or more down the line. Some great apocalypse destroys the grand order. Perhaps a civil war that uses weapons that destroy entire cities. Or a magical experiment goes wild and eats entire continents. The only safe place? The caverns below. The last people who travel there are not exiles but refugees who hope the exiles have managed to make the place survivable.
In time, the world above calms down. Another power rises. Perhaps a theocracy guided by angels and the divine power of faith. Once again it spreads, once again the world-spanning imperium sends the unwanted and criminal down to the caverns. Once again some great disaster destroys the world above. Perhaps this time demons consume the entire surface. The last and most powerful of the survivors escape to the caverns below.
Over and over. Each cycle, powerful empires send first their most powerful explorers with their most powerful weapons and armor and magic. Then they send their exiles, the criminals of whatever type, those that were too smart for the system, or too powerful for the rulers to control. Each cycle adds more magic, different magic, new ways of thinking to the populations who barely manage to survive in the still untamed wilds of the caverns.
Angels fight with demons. Elemental spirits unleashed to reshape caverns for or against this population or another. Ancient creatures who had slumbered for eons before the exiles woke them up. Abominations from the void beyond existence. All of it thrown together, shaken until unsteady, then blended into a chaotic mess and then poured unevenly back into the light and food starved caverns of the underworld.
This is the world of Exiled Future. Where magic is as common as breathing, light and food are luxuries, and stepping outside the few heavily fortified cities or well-patrolled trade routes is just asking for death by something nobody has ever imagined before.