Four Campaign Worlds & Lessons From Their Creation

So the last 4 osr campaigns I've run have all had different methods of worldbuilding, and imma talk about the pros and cons of each campaign in terms of the creation of the world itself

1. Sarkomand's Fault-80 sessions

I talked about Sarkomand's Fault earlier here but the main thing to take away is that it was very largely sprung wholesale from my brain. By which I mean it was pretty darn close to being every cliche osr hexcrawl ever.

But the intolerant monotheists were across the sea, otherwise spot on

Anyway the point is, I did all the work apart from a scant few one page dungeons scattered about. Spinning up Sarkomand's Fault took like 4 months of prep, pretty much, but I will say it was superbly easy to run- I had dungeons I knew, encounter tables I knew, it was great. Some of the further reaches were developed later, as players actually grew able and interested in reaching them, but for the most part wherever the players were, I was ready to whip something up. As for the player experience, exploration was exciting for them, because there was always something new over the horizon, and it was unfamiliar and (usually) ready to go.
Pros of Worldbuilding from the baseline of an assumed D&Dish Fantasy Setting

• Easy to run on account of familiarity and preparation
• Engaging to explore for players, without being homework to understand the setting
• Has things you the GM are excited to run, because you put it there
• Due to creating the place as a whole, everything should be easy to connect, thematically

Cons

• You, the GM, are already aware of all the cool stuff in the world, so you may find yourself wanting the players to go do xyz instead of abc
• Nothing is news for the GM
• Long, tiring preptime, doggamn
• Inflexibility- Once you've created something and tied it to other things, it's hard to replace or erase things.
• Generic Fantasy= unwelcome assumptions about how the world is that either become ingrained, or require effort to inform players otherwise

Sarkomand's Fault had pretty standard D&D fantasy stuff going on, so it had a familiar implied setting, but 'across the sea in Mercia' so the players were in OSR land rather than Modern D&D Land. This was pretty good for allowing new players to just whip up a character without needing to familiarize themselves with the setting. Which leads me to

2. Wolf Moons- ~12ish sessions

This was my first take at a very nonstandard world, plus a bunch of different thematic desires- Saresare was basically The Nightmares Underneath + all the Arabian Nights I had been reading, the central peninsula was basically all the feudalism posts from Coins and Scrolls, Vint-Savoth was basically Bloodborne, the Beast Islands (offscreen) were basically my psychic premonitions of the OSR getting into wavecrawling but also pokemon, the Wurderlands were supposed to be procedurally generated wacky content...

The Wurderlands there are roughly russia sized. For comparison, Sarkomand's Fault was the size of Cuba.

tl;dr I was way too ambitious in too short a period of prep time, and the theme was all over the place, it was a mess, AND I was using a very ill-conceived hack of the GLOG that I now shudder to look back upon. Anyway, the players chose Neth as their starting point, and this was directly after Sarkomand's fault, which was basically one long hexcrawl through an almost entirely ruined and monster-haunted wilderness.

Modified slightly from the Nightmares Underneath (TNU)

Neth, on the other hand, was less a wilderness crawl and more intended as a city crawl with lots of factions, travel between towns being more of a thing than wilderness crawls, most dungeons being TNU style psychic heist explorations within nightmares, and the overall cultural climate being based on the Arabian Nights rather than Gondor...

 

And my take on Neth from TNU really shoulda had buildings

It was madness. Madness! Not only that, but players had side-characters in other realms- there was a subplot of a character from the last world mucking about in a feudal swamp with witches, moon cults, and siege warfare, and some players with really weird almost superheroic characters doing gothic detective investigations in Draculavania, in a very Batman-style adventures. It was amusing but the confusion and unfamiliarity didn't have proper support on my end for the players to find their feet.

Pros of Making A Mishmash Of Everything You Like

• Fun to prep and work on
• Multiple campaign styles supported

Cons

• No coherent vision
• Easy to do useless prep with all those different things going on
• Difficult to get players on the same level
• Focus on 'big picture' world building antithetical to actual game focus on level 1 osr characters

The first thing that really killed Wolf Moons was that last one- thinking about the culture of whereverthefuckia ate a lot of preptime and contributed NOTHING to the pressing player concern of 'we want to get to Lenghul Monastery by going through the mountains, what do we see'
The other was a lack of an overarching goal- Sarkomand's Fault had 'Find the Orb of Omnipotence' as a sort of default any character could get behind. Wolf Moons had the bad sandbox problem of 'woah isn't this world cool guys you can do anything' without much context to apply to individual characters.

3. Rat Moons  ~25 sessions
 This was the followup to Wolf Moons, where I appropriately went smaller-scale. The players were somewhere in Wolf Moons, technically, but I focused entirely on a starting town, nearby village, and interesting local environs, and left bigger picture concerns entirely offscreen as they weren't relevant.

This place fit between the starting town and the lumber camp of Sarkomand's fault

Also, I used procedural hex-crawl generation from Melancholies and Mirth to whip this place up. Those procedures have grown more detailed and usable via click of a html coded button since those caveman days of rolling real dice and consulting blog pages, but even so, it cut down on my prep time quite a bit. Additionally, I used a modified 1-page system of Into The Odd + Maze Rats.
Finally, I focused on what really made Wolf Moons special- the system of day, night, and seasons all being randomly generated and based on Moons and one Sun rolling around the sky as well as mythical magical explanations rather than scientific ones, and the societal implications of this. All in all I think it worked pretty well, and this game crumbled due to schedules and players falling apart rather than any glaring setting issues. It also had a bit of a problem with a lack of overarching goal, but using a copper standard kept the players hella broke which was good, and the setting having things that individual characters could interact with helped players set goals of their own, like rehydrate the Dry City or marry a banjo playing giant spider or investigating the weekly Moon-caused disasters.
Pros of Procedural Generation + Guiding Principles Of Your Own +Offscreen Ontological Void

• Procedures do heavy lifting of creating things for your own inspiration to go on
• Guiding principles give direction to random content and are a constant reminder of theme
• Random content helps improv if the players go walking off the map
• GM & Players both ignorant to see what is beyond the horizon, and thus excited to see what's there, but not distracted by it as it doesn't exist yet

Cons 

• Unless you made it yourself, using someone elses RNG will require tweaks to fit vision
• Giving info to players about an unfamiliar world can turn into infodumping
• The 'void' beyond known lands can't be connected to known events until it is filled. I never did have a good idea of what was going on with that ocean isthmus Limedike was built on, and without rumors of 'what lies beyond' there was the impression there WAS nothing beyond.

The Moon world has been my favorite so far and I will certainly return there, perhaps once the current campaign wraps up.

4. Crownless Lands ~37 sessions, ongoing
 This campaign has the meta-structure of Sarkomand's Fault (BFRPG, a megadungeon beneath town, some lost artifact macguffins to find or ignore, lots of ruins and wilderness), the civilization of Wolf Moons (a main town and various small but noteworthy settlements) with the scale of Moon Rats (small starting area with dense distribution of points of interest) and a combination of self-authored, randomly generated, blatantly stolen, and player-authored content.

it's orange because it was autumn until a few sessions ago when winter hit

It's been a successful synthesis of some of the best parts of the past campaigns, though it's missing some of the per-campaign specifics that made those campaigns extra-special, and player-suggested content did a lot of work for me. There are some issues with player-suggested content that I've run into though.

1. Players don't get the thrill of discovery for interacting with something they invented, and so may get bored of their own innately familiar content
2. Players have different tastes among each other and unlike a GM, aren't necessarily thinking too hard about the ramifications of suggestions.

Player suggestions quickly cemented this setting to be a nasty place. The dominant clergy are either corrupt bureaucrats, or two-faced soul-slaving demon-worshippers. There's a spreading Blight devouring the mainland, part zombie apocalypse, part black death, part grey-goo scenario. The Lord of Hate spreads enmity and suffering among men, and they to each other. The players are forced to react lest the lands fall to ruin, and yet they are but simple murderhobos, much daunted by the forces arrayed against them.
Meanwhile posts the players wrote that were forces in favor of humanity were like, 'secret society of class-conscious robin hoods' which I liked, but wasn't the sort of thing to avert the coming doom, and these suggestions really succumbed to the first mentioned problem of players not being thrilled to investigate a secret society the players already knew about, even if the characters didn't.

The 'unlikely heroes' narrative is not really what anyone was going for, and the player who suggested this gruesome blight to begin with hasn't been to a session in a long while, but the players left with that are acclimating to the constant turmoil... though I suspect the next event if they manage to stop the blight is going to be 'Civil War' so I'm half-prepping the next campaign or at least some islands for them to flee to in the event they're fed up with this setting hell-bent on becoming another ruined civilization for adventurers to loot in a hundred years.

This experience hasn't turned me against player suggestions, far from it, but in the future I think I'd get all-player feedback about all suggestions before throwing them in to make sure there's not unexpected reactions in the setting stewpot that nobody much cared to deal with.

The Dark Side of OSR Traps

One of the worst sessions I've ever run was a modified one-page dungeon a year or two back where I repeatedly shot down player ideas to escape a trap because they didn't fit with my internal notions of how a trap-mechanism worked. In trying to get the players to problem solve rather than just roll to disarm traps, I fell into a common pitfall of puzzles where there's no easy answer- the dreaded 'Read the GM's mind to proceed' scene.

THE SCENE
A stone-walled windmill on an island in a shallow lake. The interior was a central stone pillar, a sarcophagus full of treasure that could only be be interacted with via mirrors, and two exits- a heavily barred ceiling window, and the main doorway. Oh, and some corpses of past adventurers.

Interacting with the little riddle-clue hinting of the secret coffin and sniffing about with mirrors was fun. That wasn't the issue.

The issue was when the sarcophagus was looted and made visible, the trap activated- a waterfall of flaming oil pouring from the entrance and running outside. The players were trapped behind iron bars and fiery doom.

My idea was that the windmill pumped up a reservoir of oil as it span, and had enough oil to last for about 16 hours. It could be turned off by weighing the coffin down with weight PERFECTLY equivalent to the taken treasure and closing the lid ( That level of detail and insistence on exactitude should be a warning sign that you're more invested in your own story than the quality of the game, whether you're talking traps, your character, a plot, etc). A horde of Shadows would come from the forest, slip across the lake, up the side of the windmill, and through the barred grate, converting any thieves to shadows and  returning the stolen treasure to its resting place.

That little idea of how the trap operated seems innocuous, but here's where things started to go wrong- I never hinted at the weight mechanism, figuring it would be obvious to the players that if emptying the treasure and taking off the lid from the coffin triggered the trap, it could be un-triggered the same way. An easy way to communicate this would be to have the coffin rise up as the lid and treasures were removed until it was above ground level, elevated by a pressure plate.

Here's what the players tried, and why I shot them down, and why I shouldn't have. The reasoning might seem to make sense, but here's the common issue with the rulings I made- they were based on my own headcanon of how some bullshit fantasy trap mechanism works, not rulings on how to make the session engaging and interesting.
1. Replace the Coffin Lid 
 Almost, but not quite-they needed more weight. If there had been a clue, like the pressure plate beneath the coffin sinking slightly, they would have figured it out. But they tried a half-measure that they didn't know was a half-measure and so figured the coffin wasn't the key to it all. A player even climbed inside the coffin- that weight probably should have turned off the trap, and then there would have been the problem of a player being left behind in an invisible coffin. A good scene. Or maybe it could have sunk the coffin too far down the pressure plate, indicating to the players 'aha, we need exact weights.' Ideally they then could've used the old corpses to make up the difference, but one player had rolled them all into the flames already, so they'd have to sacrifice inventory items... good stuff.
But 'nothing happens' because it didn't  perfectly fit my notion of the trigger mechanism communicated to the players 'it's not the coffin, try something  else.' And so they did.
2. Apologizing to the Goddess the place was dedicated to 
 After all, there was clearly magic going on with the invisible coffin. But I decided the goddess didn't really care if these tomb-robbers lived or died, and plus, it was a secretive goddess, not like the pushy, chatty other deities of the setting. But heck, a sepulchral voice moaning 'return the treasure' or a feeling of guilt growing when looking at the treasure would've moved things forward.
3. Filing the ceiling bars
I decided this would take too long and the Shadows would show up before they could be sawed through, because they're very thick bars designed to prevent that sort of thing from happening. To which I now say, REALLY? A player went to the trouble of having files in their pack and got told, 'no, the tools you brought to do something like this aren't good enough.' So much for 'use your inventory to problem solve,' cripes.
4. Checking for secret doors
There were none, so none were found. Pretty straightforward, right? Well, maybe not- after having all their attempts at escaping through the obvious entrances shut down, it makes a lot of sense to go 'oh, there must be something we're missing.' But the only thing they were missing was actual information on how this place worked, and I provided no way to access that information.
5. Plug the nozzles shooting oil
 The nozzles, like the coffin, were only visible when viewed in a mirror and only your reflection could interact with it. I figured the torrent of flaming oil was too powerful to simply stuff with someone's cloak.
But I could have had clogging a nozzle create an opening in the flame. Oil could back up and start bubbling from the stone pillar that contained the pump, threatening to ignite everything and everyone if they didn't flee quickly. The thief who climbed up to reach the nozzles would be in perilous danger from spraying oil and there woulda been someone holding a mirror for them as they used their hands to climb. It woulda been a fun scene.

Real time, an hour or two had passed. The initial sparks of creativity were being replaced by frustration and boredom. Player morale was breaking down fast by now. The problem player starting harassing other people for not describing their actions as completely as they did, whining and nitpicking ooc. One player declared their character was just gonna go to sleep until something happened. Another player, trapped outside, had just been throwing rocks at the windmill's sails for hours.

6. Hoist the coffin lid over their head and use that to shield them from the flames.
I mentioned that damage would still be taken from the oil on the ground, and that dissuaded this plan for the moment. What I did NOT communicate was how much damage this would be- some players probably thought it would be 'instant cremation.' Had I said '2d8 damage and you have to jump in the lake afterwards' they probably would've done that immediately, but I assumed they knew flaming oil on the ground damage and knew that's what I thought the damage would be. Don't assume or imply mechanical stuff- just say what the dice will be. That's how the mechanics of the world work, so players should be able to know those mechanics to simulate their characters making reasonable decisions. Not everything should be described via simulationist roleplay.
7. Use the rotation of the windmill to rip the bars off
 Firstly, the rock-thrower had ruined the windmill's wind-catching ability. Secondly, there wasn't wind (though there probably should've been a good updraft from all this burning oil).
This led into
8. Try to reverse the rotation of the windmill to shut off the oil flow
The player on the outside could've been rewarded for being cautious and not getting caught, climbed the windmill, been passed the rope, used his bodyweight to spin the arms, and rescue everyone.
But I decided that the windmill only rotated one way and couldn't be reversed, and anyway it had an oil resevoir that was fueling the trap so the rotation was completely pointless save to pump more oil for a later activation of the trap.
Yup. Just shutting down a player idea of how the mechanism works, without giving them any idea of how it actually works. That's bad. Don't do stuff like that, mmmkay?
9. Call me out for making bad rulings and disengage
Thuvrig Mountaincloak-"Let's just wait it out guys, we've been just getting completely denied for an hour now."
The truth hurts don't it. And it's a good idea- there's no reason you can't tell your GM 'bruh this puzzle is boring and also the suck.' Anyway, even 'wait it out' got denied, because I ruled it was night now and the army of shadows showed up.

Attacked by hordes of ethereal monsters with no magic weapons, they went back to plan#6 and fled through the oil waterfall with the coffin-shield, taking 9 damage each and escaping, the end. This was a good 3 hours of play I believe, all for 'you take 9 damage and get the treasure.' Yikes.

THE MORAL OF THE STORY
A bad trap is just like, you walk down a corridor, you rolls dice -fail to detect traps, Dave gets hit by an arrow for 1d8 damage. There's no player agency there, it's just random math.

A bad way for players to interact with traps is to simply intone 'I roll to disarm traps' upon encountering one, rather than trying to deal with it in 'the OSR way', like holding a shield between you and an arrow trap, or poking the trigger with a 10-foot pole, having an expendable goat take point, or using the trap against your enemies, or whatever.

Traps like the flaming oil windmill are good- just LOOK at all the stuff the players came up with to escape. In the beginning, it really had them thinking and asking questions and being clever and engaging with the fiction rather than the maths. The trap was scenery and context, not just a penalty applied to their HP, soon forgotten, as so many official D&D traps are.
But GMing like I GMed it, is what makes people think OSR trapfinding is stupid pixelbitching baloney. If you're ever GMing something and the only thing happening is the players getting told 'nothing happens' that's a good sign you're making some sort of mistake, with keeping them un or mis-informed, having no stakes or pressures to drive a scene, or perhaps being too concerned with your vision of 'how things are' as opposed to 'how fun things are.'

Hopefully this lesson in what NOT to do helps you see the nebulous, ever-changing form of what TO do as a GM.

The Dry City

Once called Annu Nki. A city led by the witch queen Agayba long ago. Invaders came to sack the city, and when the defenses fell, she wrought unspeakable magic to spite the invaders, preserve her people, and cheat death, all in one. She is now an immortal tree inside her old keep, and the animated roots, miles long, suck moisture from all living beings and the land, turning them to husks and the land to a dry waste. Now it is a shunned place, its only permanent resident her sorcerer lover Neroikos who works in repetitive insanity to create a body for her soul to reinhabit.

Dry Streets and hollow-eyed buildings behind a cracked and sand-blasted wall with multiple breaches. It's not so much ruined as it is deserted, and the central fortress, visible above the buildings anywhere of the city, makes you dizzy to look at it, so heavy does the weight of magic bear down.

A little hex map was useful for me to mark notable buildings/encounters in the city

Buildings have a 1/6 chance of having a path to the catacombs (12-15) burrowed by roots. 1/6 chance of loot per building(sealed jars of salt, old bronze arms and armor, stone strongboxes of jewelry, that sorta thing, otherwise just dust and old furniture), but a 2-in-6 chance of encounter per room- ransacking the city is more dangerous than profitable, as the entire population+invaders is wandering as husks or undead. You can roll for encounters per 'hex' if you're using the above city map as well.
1-Autumn Briar- A hamsterball of thorns the size of a carriage, a tumbleweed rolled around by either undead or husks contained inside. Undead seek to kill via rolling, husks roll their prison of thorns towards water sources.
2-Vampiric Roots- Stats as regular snakes, but instead of poison you save vs huskification. Usually attack from surprise by bursting out of the ground. They can't reach rooftops.
3-Witch- A member of a coven that seeks to learn ancient witchly knowledge from Agayba. As minor spellcaster with a flying device that only works for its owner- a broom, mortar and pestle, etc.
4-Husk- Sucked of their life energy and moisture save for a small amount to technically be alive. Long white hair, withered skin and sunken eyes, only motivated by thirst. 1HP but can drain water from people to huskify them on hits. A barrel of water will rehydrate them into a healthy but confused human, either a citizen of the city or one of the plundering invaders. Good reaction rolls indicate they're barely lucid due to managing to suck some rainwater recently, or too dehydrated to even move.
5-Doll- A mutant clone of the witch-queen Agayba- either cast out, escaped, or on a mission for the sorcerer Neroikos. Green eyes, black hair, long limbs, and many mutations. Intelligent and suitable to become magic-users, but only possessing incomplete souls and the naivety of a young child on account of being grown out of a vat a few months prior
6-Undead- Husks that have died, then come back. As low-tier undead that thirst for moisture but will never rehydrate, only forever thirsting. Drain people to husks. Indistinguishable from husks save for their inability to be rehydrated no matter how much water they drink, and the strength of undeath, which regular husks do not have.
7-Huskified Local Encounter
8-Conflict of 2, roll 2d6

Dark grey is subterranean

Witch Queen's Keep
Has a main entrance of great double doors leading into 1, a side servant's entrance, and 4 towers on its corners. 2-3 stories tall, roughly. The darker areas are subterranean caverns that various city Catacomb entrances lead to either 12, 13, 14, or 15 depending on which district of the city they were found in.
Main Entrance
2 promising Dolls. Cloned women with black hair and green eyes, Bronze helmets of horse and bull and wicked bardiche weapons, they flank inside main entrance. Clad in black robes, AC as leather thanks to their speed and helmets. 4HD. Naive but fierce and scornful of women, who are believed to be failed dolls.
Upper Levels
1- Tree of the Witch Queen
A large cathedral, exits aligned in each cardinal direction, with a massive black and twisted tree overgrowing an altar, with ruby droplets of blood instead of flowers.  The tree's trunk is twisted such that there appears to be a wooden woman in the trunk, with a long straight nose and high cheekbones. This is not the witch, who is dormant and immaterial, but her essence has made the tree resemble her.
Droplets heal and rejuvenate, but taking them enrages the tree. It is currently immortal and will recover from damage dealt to it unless its mortality is returned to it.

2- Servant's entrance. A lone husk still mans the entrance here, sitting on a chair inside a iron portcullis door. Has a small bell and will open the gate for those who ring a servant's bell if he's huskified. If not huskified, he will be extremely xenophobic and suspicious, since in his perception the city is still under attack by invaders, but it's only a matter of time until the roots dehydrate him again.

3- Covens Tower- Entrance from below is boarded up and warded with 13 knives. Anything passing through the door to the tower takes hits from all 13 knives thanks to the coven's Gate of the Knife spell. Witches enter via flying through the window at the top of the tower. A fairly ordinary series of rooms where the witches fly in, study things for a few days, then fly out after logging their activities in a journal. Some dried mushrooms hang from the ceiling as snacks, and there's a book on fungus identification, a map of the castle, and notes on the monsters here, as well as various plots to kill Neroikos so they can chat with Agayba's ghost in peace.

9- Tower of Dolls. Three Giant glass jars contain girls with black hair, high cheekbones, and closed eyes, floating in green liquid. There are hatches allowing access to the glass jars that could be unlocked and opened. and small tubes and funnels into which liquids can be added to the chambers, which will affect the physiology of the clones. Various small animals live in cages here.

Fran, a scarred doll watches over them. She has an inferiority complex towards her sisters as she is a failed doll kept around to hopefully one day aid a true doll to inherit the witches soul, with no chance of herself being chosen. She is a 1hd woman with surgical and child-rearing skills who will skulk in the shadows and attack, wailing, with a knife if the clones are harmed. Neroikos will be here 50% of the time during the day.

Jars of emeralds and onyx for coloration, worth a few thousand coins.

6-Tower of Power- Neroikos, Master Biomancer's study and living quarters. Doll in a servant's outfit has a veiled face and quietly attempts to take burdening items from the players and hang them on a coat-rack. She does not talk or make any attempts to communicate intentionally, basically bullied and built to be an unseen servant. Has no mouth and only one eye, the other being an empty eye socket that connects to her stomach. Entrance is locked, and he is here 50% of the time at night.

10- Tower of Moons- Astronomy notes and a large telescope are here, as well as a doll that is a giant scorpion with the face of Agayba, the ability to mimic voices, and orders to lay in ambush for the witches, or rather, any one entering the room besides Neroikos. This creature will be dispatched as an stealthy and cunning assassin to track and kill the players show up to this place, annoy Neroikos, and leave. Neroikos will be here 50% of the time at night.

4&5 Roots of the Witch Tree. Fight as 8 roots, if destroyed, hole to 11 revealed. Tangles masses of ambulatory roots that will extend to menace anyone threatening the tree.

Area Between 1 and 8-passageways up to the higher levels of 8 and down to 11. Locked, Neroikos wears key.
8- Altar of Despair-Room with 6 torchlit pillars, 10-foot balconies to the north and south with no apparent means to reach them, and at the far end, a pyramid of steps leading up to an altar, upon which a frankenstein's monster of a doll is half-possessed by the witch-Queen Agayba's spirit, but is being rejected by the nascent soul of the doll- treat as Flesh Golem if Noroikos 'activates' it. Neroikos comes here frequently to work on the project, and will be found here 50% of the time during the day.

There's a secret door on the wall behind th ealtar that leads to two branching hallways that double back and lead up to the high balconies and the towers those balconies connect to (9 and 10)

Neroikos is a master Biomancer with all spells here save Infantilize, Wave of Mutilation, because I think Infantilize is too nice for this jerk and Wave of Mutilation just doesn't seem like a proper biomancer spell, idk. You could also just give him unlimited polymorph and have him turn into a different animal each turn. He has eaten the hearts of 100 monsters and overcome the Doom, though in my game he got a bad slough skin mishap and got owned.

Dunking the Agayba Golem into the Dream and the Blood in the caverns below will allow her to be properly resurrected, but only after the tree is destroyed, and it's that aspect that Neroikos is missing.

11- Taproot of the Witch tree and a veritable forest of smaller roots that seem to tremble in the corner of your sight. If players attack the roots, treat it as a kraken that also forces a save vs dehydration on hit and also grows more roots like a hydra if they cut one off. However, one can very carefully sneak through the maze of roots without rousing their ire so long as open flame is kept away from them and you don't bump them with bare skin or spill water.

12- The Witches Dream  Puddling up in pools reflection events gone by.
Dream of the Siege- Bronze armored men and flaming catapults besieging the city. Witch laying waste from the black cathedral but growing old, until she is crumbling to dust and retreats into the Tree to suck the life from besiegers and besieged alike. You can enter the dream if you dare.

13- Place of the Skull
This is an earthen cavern with a sandy floor and two stone structures filled with alcoves the right size for a skull, and indeed some have skulls inside.
There are various names on the skull alcoves appropriate to the bygone time period. None have an X in them though this detail is not obvious and shouldn't be mentioned unless a player asks. There is a riddle scrawled in large letters in the dirt.
"There once was a man out for plunder
X marked the spot, 6 feet under
but alas, he still read on
he was naught but a simpleton
and so he was torn all asunder"

Whenever skulls are disturbed, an increasing number of skeletons (2, then 4, then 8, etc) and a massive crawling skeleton comes down a tunnel. The giant one is more like an advancing building than something you can fight, and it leaves a sandy tunnel behind leading either to nothing, or to a buried necropolis of titans if you into that. The giant skeleton is only active so long as the little skeletons are, crawling slowly forward.

Buried beneath the X in the limerick-riddle is a coffin stuffed with treasure of a skullish iconography. 10,000 silver (in a copper standard) coins and a lantern-scepter-Mace with a fragment of the Skull Moon inside, allowing to to open the lantern to reanimate corpses as uncontrollable undead.

14- Place of the Breath
3 Pools rippling at different frequencies based on the most recent statement.
"Speak, Slander Will Be Exposed and Belief and Truth walk hand in hand with Lies" is engraved on the floor.
Pool of Belief- Ripples unless a statement is/has been said that the user believes to be true.
Pool of Lies- Ripples if a statement actually is true.
Pool of Gossip- Ripples unless it the statement is about a living person
So you must say something you believe is true about someone, but is actually false to still all pools.
Once all pools are stilled, they leave and the Helm of 20 Questions is revealed. 20 shots of mind reading.

15- Place of Blood
A Flea-headed vampire demon is here.  It tells you that it will reveal the origin of each pool of blood for each amusing blood-based pun you tell it, and then it attacks, hopping forward with a spear-nose! It has 32HP, ac 15, 4HD, and can spend a turn leaping into pool of blood to heal completely, and its 1d6 probiscus heals it too, but it wastes a turn laughing at every bad pun it is told.
Bloodpool #1- The witches blood, which will restore mortality to the immortal Tree.
Bloodpool #2- Drinking it heals you a bit, but may mutate you, and if your mutations exceed your HD upon death (or twice HD in life) you instead transform into a bloodthirsty monster. I had this be accursed blood from a vampire moon but whatever.
Blood pool #3- Actually a huge vampiric Blood Pudding, content to laze about but absolutely terrifying if bothered by anything but the flea demon.
It has an infinite supply of blood down here and attacks for the sake of excitement rather than malice or hunger, and is an affable enough fellow.

Once the Tree is destroyed, the city will rehydrate next rainfall and instantly become a madhouse of ancient people renewing their war armed only with bricks and clad in rags. The wise will flee the city and become refugees, the foolish will stay and form gangs of brigands.

This site was good for, oh, 8 or so sessions in the game I ran, though a focused party could probably whip through it much quicker if it was the only thing on their plates. Originally it was for a different campaign and was full of regular undead, but the 'rehydration' and threat of being dehydrated rather than level drained was a definite improvement over the original 'ye olde necropolis' idea for sure. The ease of rescuing husks was rewarding, but the mimicry of the truly undead husks keeps players on their toes.

Six Strategic Enemies

Not to be confused with Enemy Strategos Six

thinly veiled excuse to plug Ava's Demon goes here

Anyway I was thinking there's some design space for monsters that have long term strategic effects if improperly engaged outside of maiming the PCs. Things like Shriekers are a classic example- if you annoy them, they can turn the dungeoncrawl from 'don't wake the sleeping monsters' to 'avoid the patrolling monsters.' So the goal here is less 'how does this monster affect the encounter with the monster' and more 'how does this monster affect how the players approach the dungeon'

1. Long LONG Worm
stats as- rock walls with a purple worm bite attack on the head, wherever it is
A worm, 10 feet in diameter and like a mile long, dull segments of stone-plated flesh wiggling through hallways, blocking them off entirely. It moves incredibly slowly, and is nigh-invulnerable to harm- (like a Purple Worm+Stone Golem resistances) It idly pursues meat and magic and fine metal, inching forward a meter a minute. The concern is not that it will catch you, but that its slow, stupid advance will block off key halls and trap you in a dead end with something you don't want to be trapped with. It can chew through doors  and walls, and advances 1 room or hallway each exploration turn.
If somehow split, you see each segment has teeth and a digestive tract and some semblance of a brain, and then you have two worms. Would work as either practically infinite length to make the dungeon ever more worm-clogged, or just a few hundred feet long so its movement eventually frees up space behind it.

2. Animators
Stats as- zombies, animated objects
Vengeful spirits of people the PCs have killed that possess things. Corpses are preferred, but suits of armor or clothing will do with some stuffing, or even a table, or a length of rope. Anything that can let them wobble around the dungeon and try to kill people to take their corpses. They can't leave an object until it's destroyed, and they will remember player tactics from previous encounters and try to wear the players down, pick off the weak, and basically turn every piece of scenery into a potential 'mimic' threat. And also moan things like 'you killed me you bastard now we'll kill you.' They can be 'solved' tactically by imprisoning the possessed husks somewhere, but the spirits will work together to escape and wreak vengeance. If everybody (including PCs and hirelings) killed in a dungeon returns as an Animator, you can get a fun game of musical chairs but with corpses and killer furniture going.

3. Chimerelemental

Stats as-Elemental A swirling mass of air, earth, fire, and water, a thoroughly unnatural construction of mad sorcery or possibly just an elemental orgy the likes of which Man Was Not Meant To Know. Tends to activate an scenery-changing ability, possibly clobber people for a bit if they got fresh with it, then flee, to be encountered again as a lurking threat in another area or back to the wandering monster tables until the dungeon is a hopeless ruined mass of transmuted elements. Also provides shenanigan opportunities to players.
Extra Abilities- Can be targeted against smaller instances like a player's waterskin of course but tends towards indiscriminate, unsubtle effects in 10' cubes, or in arbitrary amounts like 'one room' or 'one pond'
Gelatinize Air- Turns the air into a clear jelly, much like water but conforming handily to the locale rather than flooding away. Good for drowning people and extinguishing fires.
Sublimate Earth- Turns one floor, ceiling, or wall into a cloud of swirling smoke that could be mistaken for stone by the hasty in dim dungeon lighting, as they stay in the dimensions of the original stone. Makes pit traps, hallways to other rooms
Phlogistize Water- Turns a body of water into a bright-burning substance that'll cook people alive in a jiffy, like magma but not dense.

4. Puffbuff-
Stats as- Ogre for big ones, goblins for little ones, patches of spores as Yellow Mold A boulder-sized ambulatory puffball mushroom, possibly rolling into people like the stone ball from Indiana Jones, or possibly walking around on little legs and punching people like Dark Souls, or possibly floating around like Gas Spores. Either way, killing it causes it to explode into toxic spores that deal 1d6 damage per round you're in them, and the spores block off entire rooms as 'no go poison spore' areas. The spores infest suitable things like corpses, rations, and sofas and grow new Puffbuffs in an hour or so to repeat the process.

5. Pandora's Ox

haha reusing almost related doodles

Stats as angry bovine, but with the powers & HD of all the souls they've grazed on
A evil, rune-scarred quadruped beast of burden, obviously mutated and berserk. Survives in dungeons by devouring curses and sins instead of grass, is now bloated with corrupt energies. They are vile beasts that will trample you soon as look at you, but killing them releases their stored evil energy into the environment. NOT killing them allows them to eat the souls of other wicked monsters and become wickeder themselves.
Some possible effects of released corruption, but really it should be lore and locale appropriate

• Monsters have max hp per hd, deal max damage on hits, or always act as the worst reaction results. Boring but menacing. Have them glow all redblack hell-like
• Monsters gain additional nasty powers, such as invisibility, or giant rat watchdogs, or a long tentacle, or the power to kill a yak from 200 yards with mind bullets, or dark smoke breath
• Dungeon reveals new hazards, like flooding water/lava/gas/collapsed rocks, irate scorpions, portals to hell, dormant magical trap glyphs activating

6. Lockbug
Stats as low-threat insects
For once you can blame thieves instead of wizards for training these large beetles to use their horns as lockpicks to lock and unlock doors, and to re-arm traps, and even to pick the occasional pocket. These vermin are nigh-innumerable and lurking in the cracks in the walls, so killing them only wards them away from getting up to no good until you leave the area, so the trick is figuring out how to ward the beetles away from things you don't want them tinkering with.
What you don't discover until later is that they've also been trained to collect shiny things and will be going after treasure once you humans clear a path to it, and perhaps even before.