Epic Dungeon in the Playground
A Hellhold Asunder
A Hellhold Asunder
Ages ago, several archmages from various lands joined council to decide how to rid the world of (insert name), a colossal ancient living fortress full of dark and terrible things not of this world. Since it held such great evil, it was decided that none should dare brave the castle for fear of unleashing its evil upon the land. After long years of study and debate, they decided the best thing to do was banish the space it occupied from this world. So the circle of archmages crafted a mighty spell to tear a whole in this world around the castle and let if “fall” through. The mages gathered around the mountain upon which the castle rested and began the rites of the spell, but before they could finish, the castle reacted. No one knows what happened, but the spell only half worked. The castle still stands to this day lost somewhere in the wilderness, but only half the castle is actually there. It stands sheared in two with the other half just gone.
The half still in our world is dead, a carcass or skeleton dried up and withering away. Its dark spells and powers are gone, and nothing but parasites and scavengers inhabit it now. (Bandits, cultists, goblinoids, terrestrial monsters, roosting aerial creatures, maybe a dragon or three)
Somewhere within the castle is a hidden portal to the ghost world where the lost half of the castle now lives. In the ghost world (a demi-plane not much larger than that half of the castle) the castle is alive and strong, still containing many of the horrible things it did in ages past. It is starting to fray apart at the ends however, with different parts of the castle inhabiting the ethereal plane, the shadow plane, the negative energy plane, the far realm, or various lower planes. The castle now rests in a comatose slumber, dreaming of coming back to the prime and unleash its power in revenge.
One night, an eerie wall of mist comes down from the ghost world through the castle and down the mountain into the surrounding lands. From this mist comes some of the dark and terrible things from the ghost world portion of the castle (a group of skeletal knights riding skeletal horses, a flight of fiends, a poisonous madness upon the wind, etc.). Inside the mist, the world is warped and the stars are different. The mist only comes down at night and disappears when the sun rises, and it doesn’t come down every night, but the intervals get shorter over time until the mist is there every night, and the mist extends farther every time. Every day the land around the castle begins to look more and more like a hellscape. Once the mist comes every night, the intervening days are dark, the sun starts to look dim and red, fiery storm clouds roll out from a crack in the sky and pour acidic rain, leaving poisonous fogs afterwards, and beings not of this world being to prowl the land.
Every time the mist comes and goes, it leaves death and destruction in its wake, with slaughtered creatures, blighted wilderness, malignant curses, and other stranger things. Each night the mist brings something different and changes the land in different ways. One night, what was once a calm and beautiful sylvan forest is now a dark and haunted jungle of poisonous fungus and shrieking things, and the next night looks completely unchanged except all the beings living in the forest have gone insane with rage and begin killing each other and anyone else traveling through (just speculation, this is open to ideas obviously).
Something needs to be done.
The campaign beings with the pcs staying in a town somewhat near the castle. After a short simple (but possibly related) adventure in town, the town leader(s) come to the pcs and hire them to venture up to the castle and deal with the goblins/bandits/cultists/raiding dragon or retrieve the stolen goods/kidnapped heir/whatever.
During this adventure they get their first brush with the castle. They see that it is a long dead ruin now inhabited by creatures and beings that have moved in. They encounter a cult that knows of the castles past and seeks to bring back its evil, but probably don’t fully realize what the castle was/is. During this first adventure, or any following connected adventure, either they by accident open the hidden portal and unleash the mist, or the cult finds it and completes their rite, doing so.
After this, the next time they return to town, a (mysterious prophet) approaches them and cryptically warns them of what is to come. After this, the castle may begin to be inhabited by some of the things that inhabit the ghost world portion, or this may not happen until the mists occur every night, but either way, they need to master this worlds half of the castle, then breach the other side and deal with that half, preferably before the mists come every night and worlds begin to merge (which is bad, but in a so far undecided way).
Further on, the town the pcs started in may get destroyed or swallowed up by the mist/merging hellscape and they may need to move their base back to a bigger city closer to civilization. Time is of the essence however, so they should spend as little time as possible without the castle. As to why the pcs are the only ones dealing with the castle, they might not be (so possible alliances or recruitment of new pcs), but most people are going to be dealing with what the mist does/brings and the following hellscape and anything that comes with it. Perhaps evil all over the land/continent/world rises up as the castle gets closer to coming back, leaving the rest of the world plenty of problems to deal with. Perhaps the council of archmages became liches to guard over the castle to make sure it never comes back (again, just ideas).
The castle itself is huge. It has perhaps a dozen floors, several different towers and connecting walls, and it extends down into the mountain in a proper dungeon. It may have several sub-structures around the mountain, perhaps connected by tunnels. There may be a floating portion of the castle, or several floating towers, or something similar. The dungeon may connect with an ancient dwarven mine, or the underdark, or both.
The castle is sheared in two, meaning that at the shear the castle may be open to the elements (imagine if half a tall building collapsed). Some parts of it may have collapsed, leaving big piles of rubble and maybe some of the lower floors have buried because of it. Perhaps the cult or other inhabitants have added superficial fortifications to the ruins, maybe blocking off the open parts of the shear or adding fort-style wooden walls around the bailey, it’s all possible.
The castle was once a living thing with much great evil, so it should have “organs” – for example, a now non-functional great crystal heart, waiting to be revived. Maybe the heart has some evil power, maybe reviving it accomplishes some goal (like opening up new parts of the castle).
The ghost world portion, and the prime portion once the castle re-merges, will be “alive” (but not “awake”), and can react to what the pcs do, meaning it might close off some areas, call security, etc., and will be much more powerful in this regard if it ever fully awakens.
What the castle is, what inhabits it, and what its ultimate goals are is up for debate. Mainly, the pcs want to keep the castle from re-merging, re-awakening, and kill it (or its controllers, whatever).