Millennia ago, the celestials built a great dungeon. Within it, the celestials erected mammoth walls, locks, doors, and traps. It warded off those who would enter, but also walled in those who would escape; it was a dungeon larger than any ever known. Within it the celestials imprisoned hellish undead, demonic beasts, and fiendish monstrosities.
It was a perfect construction. Before the first dwarf would ever be born to sculpt stone, angels and celestials carved a work of art that no one would ever see. Perfect in every detail, it would stand inviolate for thousands of years. But none can say why the gods ordered demons and liches to be entombed and not destroyed. And in time, the gods would rue their mercy.
Centuries passed and the tomb stood unchanged. The champions who protected humanity from the liches and hezrou and pit fiends stood vigilant guard, ready to spend eternity in honor of the gods. But time passed over the celestials. The world forgot its heroes. Even the heavens turned a blind eye to the angels, inevitables, and guardians serving out their destiny entombed in the earth.
No one is sure how much time passed; history has forgotten the quest of the celestial heroes. One thing is certain: the earth is an unforgiving mistress and her power cannot be held back.
An earthquake cracked the foundation of the dungeon. The rift spread for miles, nearly ripping the dungeon in two. Into the rift flowed magma, but it did not boil the occupants of the dungeon. The temperature rose beneath the earth, and the cold home of the unliving became capable of sustaining life.
It was not long before the derro came. They made their homes among the rocks and tombs. Many of the traps set throughout the dungeon had been triggered, and creatures that the celestials had sworn to entomb eternally had escaped. In time, more subterranean creatures would make their homes in the rock. All the while, celestials contained the truest evils in the heart of this prison.
Then another earthquake pushed jagged earth through the dungeon floor, destroying a large portion of its foundation. Driders and drow dug beneath the rock and rubble to make their homes. The same earthquake tore a hole in the ceiling, and flying beasts like the arrowhawk and belker perched on the highest plateaus of their underground sanctuary.
As the dungeon's population grew, so did the celestials' fear that mankind would be destroyed by its inhabitants. Two titans made the long journey to the mountains and stood guard at the entrance and exit to the tombs, in eternal vigil.
Time passed. Few monsters made the climb down from the surface or pushed up from below. Only the strongest made a home in the debris of the prison. Eventually, more derro came, bringing other diggers with them. When they required more room, the derro dug deeper.
The burrowing derro caused a massive landslide, bringing thousands of pounds of earth and millions of gallons of water crashing in. Hundreds of derro and duergar died and the survivors moved deeper into the dungeon. Enlisting every manner of enslavable race, the derro crossed a giant chasm and started digging again.
Within the last hundred years, the dungeon has seen unprecedented activity. Every manner of creature lives in the dungeon. An elder treant and its elven entourage have made a refuge in the westernmost portion, while the undead have remained entombed at the northernmost point since the dungeon's creation. The core of the dungeon show the greatest sign of conflict, with angels and devils waging a war that must end poorly.
Others have learned of the great dungeon, they know little of it's history or true nature, but they have found a way to make it worth their time. Stealing away "players" from an alternate dimension they entertain themselves by sending in groups to explore the depths and they watch, betting on the outcome of battles and gloating over which of their "champions" live and die...