I'm brainstorming ideas for a custom adventure path, though I'm not expecting to actually finish it/write it up/use it.
For this idea, I was thinking of an overall goal of having the players construct a dungeon. Now, the question is, why would they? My first thought was to house some Sealed Evil in a Can, or house an evil artifact, but in either case, their time would usually be better spent actually trying to destroy the thing. (Though it occurs to me that perhaps the method of destroying the artifact involves it sitting in some specific environment for a year and a day or something.)
So, what justification could you give the players to encourage them to actually build and stock a proper dungeon, rather than just use/abuse/destroy/bury the MacGuffin?
Barad-Dur was built not as a fortress to keep people OUT of Mordor, but as a barrier to keep things IN.
So you can always go with the idea that there is some mysterious gateway into an evil universe that is too powerful to seal or destroy and so needs to be protected.
There are loads of examples in myth and legend of powerful forces building prisons instead of simply destroying their occupants. In some cases (e.g. Prometheus) it is because the goal is to punish the occupant, not to destroy them. In other cases (e.g. Tartarus) it is because the occupant is beyond the power of the victors to utterly destroy, and imprisonment is the best they can do. In still others there might be hope for rehabilitation.
Barad-Dur was built not as a fortress to keep people OUT of Mordor, but as a barrier to keep things IN.
So you can always go with the idea that there is some mysterious gateway into an evil universe that is too powerful to seal or destroy and so needs to be protected.
There are loads of examples in myth and legend of powerful forces building prisons instead of simply destroying their occupants. In some cases (e.g. Prometheus) it is because the goal is to punish the occupant, not to destroy them. In other cases (e.g. Tartarus) it is because the occupant is beyond the power of the victors to utterly destroy, and imprisonment is the best they can do. In still others there might be hope for rehabilitation.
On that last note, now I want to stat up some Death Gate Cycle.
|
If the location has something special going on, such as a magical pool that provides healing abilities or dispenses oracular advice or a nexus of planar energies that can be tapped to enhance certain types of magic or a conflux of ley lines or a connection to the Plane of Shadow that enhances some stuff and penalizes other stuff, a group might be strongly motivated to build a citadel or dungeon around it, so that they can hoard it's magical resources for their own advantage, and keep rivals from destroying it or tapping into it or turning it against them.
Whatever advantages are gained from it, even if they are only defensive (scrying magic doesn't work within the area, making it a safe place to hide out, and after the party wipes out the previous evil cult residents, they decide to keep it to prevent their own enemies from scry-and-frying them), will be stationary, and in some way vulnerable, giving the group good reason to fortify the position, and leave an assortment of traps and guardian creatures to protect it when they are out and about doing other stuff.
Other benefits could be more or less temporary.
A portal to the plane of fire could add +1 CL and +1 DC or +1 hp/die to fire spells prepared within it's light, which limits the bonuses to when they can prepare spells (and only very specific spells, which the GM can counter with a simple resist fire spell) in their secret base, and won't necessarily be useful when they are in the next county, three days later, having to re-prepare spells.
The Drow of previous editions had magic items that didn't work outside of their special domain. A total ripoff of this idea could be a quasi-stable source of shadow magic that can be used to enchant certain types of magic items at a much reduced cost, but they are only usable within a half mile of the shadow portal and are depowered by the touch of sunlight. The party has a reason to secure this source of power, and, unless they are attacked within their 'dungeon,' it doesn't significantly affect their own power level.
| 2 people marked this as a favorite. |
Reasons for PC's building a dungeon:
1) it's a dungeon. Like the word it comes from donjon, meaning a prison? Maybe they don't have to KILL every BBEG but they want to capture them.
2) To weaken a foe. It's hard to kill a god; imprison it somehow though and use said prison to leach off some of it's power... now you've got a chance.
3) competition?
4) going w/the prison thing plus an old convention from 1e/2e (don't know if it works this way in PF): killing a demon on the material plane just sends it back to the pits where it later re-spawns. Killing it on its native plane is darn near impossible. IMPRISONING it here takes it out of circulation.
5) priests of Abadar - think "Gringots" from HP series.
6) a REALLLLLLLLLLLLLY big wine cellar.
7) a mine complex
8) building off the last one...a DWARVEN mine complex!
9) Day one of first level, the rogue shouldn't have pressed that button labeled "doomsday"; now the sky is scorched and evil constructs have come to use people as fuel. The party is burrowing down, near the core, where it's still warm; they call it Zion. Maybe if the party lives long enough they'll even see it for themselves.
10) the party's either evil or monsters.
11) to hide their shame - lycanthropy, forced undeath, witch's curse, centaurism, whatever.
12) Only by building a labyrinth, to the exacting specifications of the God/Aberration/Entity (whatever) can they draw together all the ley lines at it's eventual core and thus power a machine housed there which, once activated, will save the planet.
Now...GO GET A D12!
This may be slightly off topic, but a friend and I were talking about Pathfinder one day and we came up with the idea of a dungeon salesman.
Basically they were the the authority on dungeon design, and sold and installed everything from basic pit traps to planar rifting mines, and full blown custom dungeon design and construction.
They had a facility that had two or three sample dungeons that they would show to aspiring BBEGs and their partners.
Stuff along the lines of:
"And now if everyone will step behind the safety glass, we will now demonstrate the flame mines."
"Cynthia, release the goblin."
It was us just goofing around but it was an extremely fun conversation.
Some ideas:
- They were commissioned to build the dungeon as "a tomb" for a wealthy and private individual. They may begin to notice the dungeon is especially weird for a tomb, and the guy who hired them doubly-so. If they're smart, they may learn that they're going to be sacrificed when the job is done, in order to keep the dungeon's secrets secret.
- They were hired by a very high level wizard who is going to "retire" to the Astral Plane forever. He needs a vault in which to hide his body. In return, he'll pay them well, and they get to use the vault as a hideout or whatever.
- It's a time capsule, meant to store artwork and literature for the sake of future generations, so it's gotta be secure and water-tight. It was commissioned by an organization of wealthy philanthropists.
- The evil artifact that they're tempted to destroy, actually holds a piece of a soul of a very important Outsider, one who knows things that certain kings will want access to in the future, so destroying it is out of the question. (The information could also be the "goods" on rival kings or other rulers, and the artifact is being locked away for use in the future). Naturally, being trusted with such a thing is going to cause all kinds of interesting problems for the PC's, ESPECIALLY when they're told to "make the dungeon look like a simple cakewalk for some young adventurers, so they'll come in, kill some stuff, get some minor treasure and then leave. Keep the dungeon stocked, too." Which will give the players something to do.
- It's a temporary prison for a notorious criminal, who is set to stand trial as soon as the rest of his nefarious crew is rounded up. He's a clever bastard, though, so keeping him secure is going to be a challenge.
Used this one in my game a few years back:
BBEG (powerful 'immortal' creature with serious Druid capacity) has gone wacko. It, or rather, It's life force is what keeps the local area from slipping into a zone of raw stuff of chaos. Add to this the fact the creatures from that area are scared witless of it and are powerful enough as mages to monitor It's life force. Siting and crafting a fortress/tomb kept all three of my power gamers knotted for most of a year.
The usual reason is because it's a security system, either to guard something from thieves, or to keep something from getting out. Occasionally both.
Other sorts of dungeons include ruins which were not designed as dungeons but effectively become them after something like a war or a natural disaster. Take a temple. Bury it in a mudslide. Have the ruins haunted by an assortment of natural creatures moving in and undead that resulted from the grisly deaths of the dead monks. Or have an earthquake, a volcanic eruption, a flood, a plague, a famine, or just a plain old war ruin some formerly fabulous city.
6) a REALLLLLLLLLLLLLY big wine cellar.
The Bard's Tale. Nice.
Just to toss in something different: To test other adventurers! Seriously, it's a proving ground for the local chapter of the Pathfinder Society (or whatever faction you like). At least, that what they told the PCs. . . .
There's some good suggestions here.
I'm trying to figure out which options work best when thinking about things from the perspective of a potential adventure path.
- A mobile/portable MacGuffin allows an exploratory element as the players try to find the best site for the would-be dungeon.
- An actively hostile MacGuffin would potentially just kill the low-level party at the start of the path. (An insane MacGuffin might not.)
- Having the MacGuffin tied to higher-ups raises the question of why low-level adventurers are given the task of guarding it. (Maybe they aren't. Maybe the king still guards it, but has tasked the players with finding a better way.)
- A vortex/gate that would let in stuff that would need to be sealed away would also likely kill a low-level party. (Maybe they know the vortex will form here over the course of a year or two. The higher-ups might be busy or just don't believe them.)
This leaves three categories of MacGuffin: An object, a person/creature or a place. Let me try and sort out a few points.
- For an adventure path, it should start with the players at levels 1-3. (Most start at 1, right?)
- The MacGuffin should show up sometime within the first book, but not necessarily at the very beginning.
- So the players should be at most level 5 when they first encounter the MacGuffin. They may not actually have possession of it at that point though.
- For there to be much ongoing conflict, there would probably be several factions vying for control.
- As for the dungeon, in my mind, a proper dungeon (as opposed to say haunted ruins) could only exist if the builders (or whoever hired them) wanted to be able to get in and out. Otherwise, they'd just try to completely seal it. This also limits what the MacGuffin could be.
- The final book would be the climax where the dungeon they built is put to the test. (Otherwise, what was the point of building it?) Of course, other things would have to be happening to keep the players engaged.
I'm sure I'll come up with more later. Anyway, none of this seems to rule out any of the three categories. Any thoughts?
|
The "dungeon" itself is an enormous magical circle or other such symbol that, once complete, will destroy/seal the BBEG or force that's the primary threat of the campaign.
Every hallway and room is a part of this eldritch diagram, and every time a new piece is added the walls of reality crack or move a bit. THINGS find their way through, minions or "fingers" of the BBEG itself slip in and try to either sabotage this work or to corrupt it towards their own ends.
The PCs are doing the only thing that can stop the BBEG, but they'll be playing with fire the entire time.
1) The PCs are helping a town/populace prepare for an assault, or possibly helping them get set up to defend themselves for a longer period of time than the PCs are going to be in the area. Perhaps there's only one real approach to the region that can be used to funnel invaders, or you could set up a bolt-hole for the folk: anyone trying to get to them must pass through the dungeon the players construct.
2) The players are working for some organization that wishes to test new members, or whatever. Perhaps it's a religious order, or a god seeking new champions, and the point of the dungeon is to test applicants.
"we have to keep building. If we stop, we're all doomed" its sort of a living version of S. King's Rose Red.
Maybe it's nothing more than a place to live. Halflings (before copyright infringement) lived in a hole in the ground. Not a dank, dirty, sandy hole, but a dry, warm, cozy hole. It was a "Halfling" hole, and that meant, comfort. (my apologies to the Tolkien estate.)
Anyway, weirder things have happend. Way, WAY back in the ancient days of High School, one of my players was a Dwarf named Badaxe. The player concieved a grand hall when he reached teen levels, but he didn't just want a hall and personal quarters. So he sketched up (yes, by hand on graph paper children) a 3 tiered "dungeon" going up instead of down, leading to his personal grand hall and quarters for all the PC's near the top of his mountain.
"An Oubliette is a place you put people when you want to forget about them" - Hoggle
Come come come now Hogbrain! (that's Hoggle sir) Who could ever love a wretch like you?
A TOUR DE FORCE for the Great White Duke and all the muppets! I can't believe that's where Jennifer Connely STARTED. I also can't believe anyone besides me even remembers that movie anymore. Truly old school...
"An Oubliette is a place you put people when you want to forget about them" - Hoggle
Come come come now Hogbrain! (that's Hoggle sir) Who could ever love a wretch like you?
A TOUR DE FORCE for the Great White Duke and all the muppets! I can't believe that's where Jennifer Connely STARTED. I also can't believe anyone besides me even remembers that movie anymore. Truly old school...
Labyrinth. A lot of people remember that one. I've watched it my kids several times courtesy of the DVD...
Okay, so you're trying to write an Adventure Path in the style of Paizo, correct?
I would write up some Campaign Traits that make the PCs be acolytes to an old order in an isolated location. There are very few members of the order left, and they've been there for so many generations that no one knows their true purpose - to maintain the seals on an ancient evil artifact.
In the first adventure, a freak event - weather, a raid, an occupying army - kills off the last full members of the order (or moves them elsewhere…), and leaves the PCs trapped alone in the catacombs below, where they accidentally unseal the artifact. They also find a chamber with some vague notes about the artifact and the existence of other orders. The artifact itself is not destroyable, nor is it (currently) movable, so the PCs are forced to leave it there. It may project a miasma of evil at this time, but little or no other effects, as it has just re-awoken.
The second adventure finds the characters crossing new lands to find other branches of their order. The first place they locate has been long deserted, but is occupied by a group of dwarves currently repelling a strange army coming from the depths (there is one fragment of the artifact at each order location). When the PCs help the dwarves successfully repel the attack, the dwarves thank them and direct them to an active branch of the order.
The PCs encounter trouble with this new branch of the order in the third adventure as the artifact continues to send out wave after wave of strange and deadly foes. Eventually, the PCs convince the order elders of the truth of their words, and they are granted access to the knowledge they need to be able to stop the artifact. But, they're going to need help to do it. The catacombs the artifact(s) were sealed under are no longer sufficient to hold it - separating it into four pieces was only a stop-gap solution, giving the order a few centuries to properly contain it. The PCs also discover that, even if they had not broken the seal, it was weakening and would have broken on its own. The PCs return to the dwarven community to request their aid in building a new prison for the artifact, using the notes from the order's archives as a blueprint.
In the fourth adventure, the PCs seek out special materials the dwarves need to build the dungeon, as well as the aid of other communities - each race or culture might have a different way to build dungeons, providing unique benefits. The artifact is actively hunting them now, so the PCs encounter elite fiends/aberrations of increasing power as they seek to contain it. Additionally, the workers in the dungeon will need help to fend off attacks as well.
The dungeon construction is well under way in the fifth adventure. However, the PCs must discover the location of the fourth and final artifact piece before it can be completed. This last location is very well hidden. When the PCs finally locate it and approach, they find that the artifact has filled it with a host of its horde, in a mockery of the order. The PCs must find an inventive solution to cut a path through their enemies.
In the sixth adventure, the PCs have the dungeon mostly completed, but must find a way to move the artifact into its new home. It has grown much more powerful in the mean time, so special care must be researched and taken in moving it. And when the fragments are brought closer together, the artifact grows in strength. At the conclusion of the adventure, the artifact pieces are brought into the central chamber, and the PCs must face its most terrible power before completing the ritual and silencing it… hopefully for good.
On a theme of a gate to a bad place, they might first send through the livestock. You can mark a planetouched rat so you can scry on it. Repeating traps disarm-able from our side would be desirable. If you are dealing with a door to the netherworld, where all things dead and destroyed have a second, undead life, would be dungeon worthy. A pit filled with holy water, would be a nice trap.
Just to toss in something different: To test other adventurers! Seriously, it's a proving ground for the local chapter of the Pathfinder Society (or whatever faction you like). At least, that what they told the PCs. . . .
...and maybe it's in fact the training ground of somebody else.
*cough* AVP *cough*
My party built a dungeon to contain a monster. It was of the "Only one in existance at a time" variety and we knew that if we killed it, another one would pop up shortly after, so we opted to keep it alive and imprisoned.
Didn't read all the way through, so if something like this was mentioned sorry but...
When you're a low-level adventurer you occasionally will have higher level NPCs as your patrons, right? In terms of an adventure based around this, higher level PCs would be a part of that group of patrons for a low-level NPC adventuring group. In many campaigns, too, the PCs eventually get to a point where all hell is breaking loose; the planets are aligning (crap), the portal opened (jeez...), the king has imprisoned all detractors(jerk!) and the NPC adventurers escaped. Whatever it is, the PCs are part of the group building the hideout or base of operations now that there are demons running amok or the king's guard is trying to arrest everybody that doesn't kowtow.
Basically, the adventurers could be the deus ex machina for a group of NPCs who may be in over their head or weren't really the ones destined to save the world; when they get beat up the PCs are the ones who were really supposed to step in and carry the day.
magic radiation makes being outside for long dangerous thus all homes have to be build below ground, and your heroes like a biiig home.
Your heroes best friend was the dwarf king who died recently, he needs a tomb.
The annual "best dungeon"-competition held by the dwarfs, the winner can marry one of the many bearded daughters of the king.
A high tower needs a deep foundation. And then you might as well use those thick subterranean walls for something.
Okay, so you're trying to write an Adventure Path in the style of Paizo, correct?
I would write up some Campaign Traits that make the PCs be acolytes to an old order in an isolated location. There are very few members of the order left, and they've been there for so many generations that no one knows their true purpose - to maintain the seals on an ancient evil artifact.
In the first adventure, a freak event - weather, a raid, an occupying army - kills off the last full members of the order (or moves them elsewhere…), and leaves the PCs trapped alone in the catacombs below, where they accidentally unseal the artifact. They also find a chamber with some vague notes about the artifact and the existence of other orders. The artifact itself is not destroyable, nor is it (currently) movable, so the PCs are forced to leave it there. It may project a miasma of evil at this time, but little or no other effects, as it has just re-awoken.
The second adventure finds the characters crossing new lands to find other branches of their order. The first place they locate has been long deserted, but is occupied by a group of dwarves currently repelling a strange army coming from the depths (there is one fragment of the artifact at each order location). When the PCs help the dwarves successfully repel the attack, the dwarves thank them and direct them to an active branch of the order.
The PCs encounter trouble with this new branch of the order in the third adventure as the artifact continues to send out wave after wave of strange and deadly foes. Eventually, the PCs convince the order elders of the truth of their words, and they are granted access to the knowledge they need to be able to stop the artifact. But, they're going to need help to do it. The catacombs the artifact(s) were sealed under are no longer sufficient to hold it - separating it into four pieces was only a stop-gap solution, giving the order a few centuries to properly contain it. The PCs also discover that, even if they had not broken the seal, it was...
This is the kind of thing I was thinking of. My current idea is something along these lines (borrowing some ideas already posted):
Book 1 - The adventurers capture some bandits near the capital city. They know that they can collect a reward in town. When they go to do so, they're invited before the king where he explains that he needs adventurers to help locate a suitable site to build a special vault. (Lots of time spent studying maps and running around the country.) The books ends with the party making a case for their favorite location. (Epic skill check to convince the king to build it under the capital.)
Book 2 - A month later the king sends for the party again. After much debate, a site has been picked (the party's input might have changed which). Now he needs brave adventurers to first secure the area and then assist the royal architect in coming up with methods of keeping people out. Around now it's finally revealed what they're burying (which I still haven't quite worked out). During this time, certain objects/materials will need to be collected to help the process along. The players can also spend time investigating rumors of items or people that could improve the situation. The book ends with the breaking of ground at the site.
Book 3 - The dungeon is being built, but other factions vying for the item are starting to feel the pressure. The players must prevent sabotage and theft from slowing the process down too much. They also get a chance to help design and build some of the sections of the dungeon if they want.
Book 4 - The dungeon is taking shape, which happens to be an eldritch one. This is causing things to leak in to this plane. The chief architect says this is an expected part of the process and will go away once the dungeon is completed.
Book 5 - The players discover that the chief architect is a mole from one of the factions that want to use the artifact for evil purposes and completing the dungeon as is will release something horrible. The players must stop the architect and then try and fix (or find someone to fix) the design.
Book 6 - The dungeon is finally finished. Unfortunately, the artifact is still outside. Now the players must trek through the dungeon they helped to create to seal it away. Fortunately, they know how to bypass many of the traps. Unfortunately, the warped space is causing some problems.
(Well, something like that anyway. BTW, are all APs 6 books?)
A benevolent artifact, but it only serves a single purpose, to kill some great big evil that shows up every 1,000 years (or whatever long period you like).
The players start off as part of an army or force that is fighting some ultimate evil. They're level one, so they aren't the ones in charge, but they get to see the king/prince/hero's/etc. They're side is losing, but there's rumors of some advantage that will turn the tide.
Book 1: some adventuring, being recruited/joining the army, some examples of how evil the evil thing is.
Book 2: build up to the "final battle", then the actual battle. The battle destroys the majority of both armies, but the players survive and find the macguffin.
Book 3: the players, probably thinking they need to give the item to someone who knows more than them discover that there are forces who want to destroy it, so the evil can come back sooner and win. They get attacked by several groups giving them the impression that there is almost no one they can trust.
Book 4: trying to find out more about the item, the players research it and find out what will happen to the world if it's destroyed/never found. They have to start looking for a place to hide the item.
Book 5: Gathering resources and starting initial construction. Dealing with problems and delays.
Book 6: continuing construction, plus the have to find a way for the hero's from a 1000 years in the future to find clues that will lead them to the item.
Dotting, because this just gave me an idea for the beginnings of a campaign...
*wrings hands evilly* Muwahahahaha.......
Basically, the PCs are tasked with collecting the resources, and manpower to build a dungeon. Then the players get to DESIGN the dungeon, under the assumption that it will house many BBEGs that they and other heroes capture. Eventually it will culminate in all of the BBEGs teaming up, over running the dungeon, and taking it under their control... Eventually they're going to try and break out, and succeed. It's at this point that the party is sent in to quell the rebellion, stop the plethora of BBEGs they inadvertently teamed together, and retrieve the macguffin that just so happened to spawn at the nexus of leylines centered in the final chamber of the Dungeon...
So it will essentially be a dungeon crawl/tower defense (dungeon defense) game!
Edit: Having skimmed over the rest of the thread, I like the idea of it sealing an extradimensional horror. May have to work that in.
|
|
It's at this point that the party is sent in to quell the rebellion, stop the plethora of BBEGs they inadvertently teamed together, and retrieve the macguffin that just so happened to spawn at the nexus of leylines centered in the final chamber of the Dungeon...
If the BBEGs are themed after the Fatal Five (five independent villains accidentally assembled into a much greater threat by a team of heroes...), so much the better. :)
It's at this point that the party is sent in to quell the rebellion, stop the plethora of BBEGs they inadvertently teamed together, and retrieve the macguffin that just so happened to spawn at the nexus of leylines centered in the final chamber of the Dungeon...If the BBEGs are themed after the Fatal Five (five independent villains accidentally assembled into a much greater threat by a team of heroes...), so much the better. :)
I'm not quite familiar with them, I'll have to check them out. Might make for a nifty little easter egg to.... DC fans, if this is the correct wikipedia page, lol. Thanks for the idea for a first team of BBEGs. Gotta say though, I'm liking the idea more and more as I scribble down notes here and there.
|
|
I'm not quite familiar with them, I'll have to check them out. Might make for a nifty little easter egg to.... DC fans, if this is the correct wikipedia page, lol. Thanks for the idea for a first team of BBEGs. Gotta say though, I'm liking the idea more and more as I scribble down notes here and there.If the BBEGs are themed after the Fatal Five (five independent villains accidentally assembled into a much greater threat by a team of heroes...), so much the better. :)
My notion for a D&D-ish Emerald Empress would be a Sorceress who has a special technique that allows her to take the severed head of a dead arcanist and make it into an orbiting servitor, from which she can cast the spells it died knowing as from a scroll. Once the spells it knew (or the spell slots it had left, for a Sorcerer) are depleted, the head loses it's power. At any time, she'd have three or four severed heads floating around, with a random selection of spells remaining.
In 3.5 terms, I'd make Validus a half-illithid storm giant, Tharok a half-iron-golem warlock or wizard, Mano some sort of undead monk with a deadly touch, and the Persuader an ogrekin antipaladin or blackguard with a vorpal spellsundering adamantine greataxe.
Dotting, because this just gave me an idea for the beginnings of a campaign...
*wrings hands evilly* Muwahahahaha.......
Basically, the PCs are tasked with collecting the resources, and manpower to build a dungeon. Then the players get to DESIGN the dungeon, under the assumption that it will house many BBEGs that they and other heroes capture. Eventually it will culminate in all of the BBEGs teaming up, over running the dungeon, and taking it under their control... Eventually they're going to try and break out, and succeed. It's at this point that the party is sent in to quell the rebellion, stop the plethora of BBEGs they inadvertently teamed together, and retrieve the macguffin that just so happened to spawn at the nexus of leylines centered in the final chamber of the Dungeon...
So it will essentially be a dungeon crawl/tower defense (dungeon defense) game!
Edit: Having skimmed over the rest of the thread, I like the idea of it sealing an extradimensional horror. May have to work that in.
I started with the idea of giving the players time and excuse for designing and building a dungeon, but coming up with a MacGuffin that the players would reasonably have sole control over (especially starting at low levels) is tough, and generally if they weren't in control, whoever was would want some control over the design.
Also, a slight tangent but there were a lot of suggestions for things like underground houses and such. While cool, would those be dungeons? If so is there another word for the kind of dungeon full of traps built for the purpose of protecting someone/something?
Dotting, because this just gave me an idea for the beginnings of a campaign...
*wrings hands evilly* Muwahahahaha.......
Basically, the PCs are tasked with collecting the resources, and manpower to build a dungeon. Then the players get to DESIGN the dungeon, under the assumption that it will house many BBEGs that they and other heroes capture. Eventually it will culminate in all of the BBEGs teaming up, over running the dungeon, and taking it under their control... Eventually they're going to try and break out, and succeed. It's at this point that the party is sent in to quell the rebellion, stop the plethora of BBEGs they inadvertently teamed together, and retrieve the macguffin that just so happened to spawn at the nexus of leylines centered in the final chamber of the Dungeon...
So it will essentially be a dungeon crawl/tower defense (dungeon defense) game!
Edit: Having skimmed over the rest of the thread, I like the idea of it sealing an extradimensional horror. May have to work that in.
I started with the idea of giving the players time and excuse for designing and building a dungeon, but coming up with a MacGuffin that the players would reasonably have sole control over (especially starting at low levels) is tough, and generally if they weren't in control, whoever was would want some control over the design.
Also, a slight tangent but there were a lot of suggestions for things like underground houses and such. While cool, would those be dungeons? If so is there another word for the kind of dungeon full of traps built for the purpose of protecting someone/something?
Vault.
Bumped by popular demand.