It's split into several disconnected sections
Let's start with Carroll's quarters, as the players may be invited here for dinner, chitchat about the surface world, and generously offered human sacrifices.
M1 is an entrance room and lounge, where the players can sit in stuffed leather chairs and converse urbanely about Under and Overmen surface politics, or perhaps about the monster heads mounted on the walls. There is also a locked door that can be opened by servant's keys, though the outer door is only accessible via Carrols key. A chandelier of hollow glass bottles each with a single skullmoth maggot and a small piece of meat is here, providing illumination. There is a cabinet with several bottles of vintage Underliquor that would be worth a lot to the right collector.
M2 is a dining hall/chapel to the God of Bloody Gold, and it is here that the players will eat dinner and have whichever servant brought them their food sacrificed on the table by Carrol after. The table is a big plank of wood over a stone altar, and this much wood is quite luxurious for the Underrealm.
M3 is a place where artistic pieces like urns and paintings are kept. There is usually a servant standing by here ready to attend to Carroll and keep the art free of dust. The art pieces can be looted and sold for good amounts, though they are massive, bulky, and distinctive, and getting them through town without being challenged would be an adventure.
M4 is Carroll's bedroom. There is a wardrobe full of grand robes, the bed is of spidersilk and bat fur, and there is a big clawed-foot bathtub that has some Mephits(Elemental Imps basically) trapped in crystals so that their amusing contortions can entertain Carroll and their powers of ice, steam, and mud can be harnessed to draw various baths. His multi-volume diaries are full of banal social interactions, brutal abuses of power, and handy bookmarked notes on dealing with the entities in A2 such as the chained ettin and the oracular medusa.
S1- This is the kitchen and pantry. A Salamander is kept in a fireplace to provide heat for cooking, and there are usually a few servants here. At the back of the pantry is a locked door that lead into A2.
S2- These are the slave quarters, where 'disposable' servants are watched over by long-term servants. If slaves please Carroll they may escape their inevitable fate and become long-term servants, otherwise they'll all end up as sacrifices to the God of Bloody Gold. d3+1 slaves and d3+1 servants are here usually.
S3- Servant Quarters. Long term servants stick around here, and greet the messengers of the Underking when they arrive. There is a secret one-way door at the end of the long, large hall that can't be entered directly from this side and appears as a normal wall, and the eastmost room has a giant chain winch that can be rotated to retract the chain. This takes nearly an hour, but will drag the Ettin of A2-5 back into A2-14 if fully wound up
The slaves and servants represent the d4+2 lantern archons for they have skullmoth lanterns, the d3+1 derro, formian workers, gnolls, trogolodytes, and yuan-ti purebloods for they are humanoids that can be negotiated with and played off each other, the ghasts for the slaves and servants will fight to incapacitate and shackle the players like paralysis, and like harpies because while no charm effects or flying is in play, the slaves are a lure into greater danger, as if you try to rescue them (or claim them as servants) you will draw the ire of the Undermen.
Well, now for the more monster-zoo areas.
A1 has 6 rooms and is Carrolls 'playground' where he uses his bloated MaxHP to play with his pet monsters, who would tear a regular human to shreds.
In room 1, we'll combine Displacer beasts and Howlers into Cu Sidhe, dogs of the Fae. The dogs are large green hounds the size of a pony and are kept in iron manacles, which hurts their flesh and is the only metal that can harm them. They are ill-tempered, bored, and restless, and capable of various faerie tricks like going invisible, mimicry of the human voice, and ventriloquism. They speak Elvish and were captured for Carrolls entertainment by the Annis a few years back, and could be helpful allies if released from their chains and negotiated with properly, but they will be distrustful of humans.
Room 2 contains another vaguely fey creature, a Will O' Wisp. It is trapped in a contraption like a large puzzlebox of silk screens and iron bars, and shifting this contraption allows one to make various tricks and shadowpuppetry of light and shadow. Mastering the shadowbox is an artistic skill, and it would be an excellent addition to any theater company or illusionists repertoire.
Room 3 contains a pigpen. This replacement for 'Wereboar and d3 boars' is actually a prison for those who displeased the Underking, and the swine were once undermen and women who were transformed into pigs for some crime or other by the Annis. They cannot speak but display their human intellect still, and wish dearly for vengeance. They have mud from room 5 to wallow in, though they refuse to do so as a means of retaining their human identity.
Room 4 contains my combination of d4+1 shocker lizards and d3+1 locust swarms. A hive of Lightningbees hangs here, behind delicate glass doors. Their honey and stings electrocute those who experience either, and while the sensation is quite unique, ordinary people will have their heart stop from all the HP damage. Carroll and his divinely inflated HP, however, views the entire thing as a delightfully decadent experience.
Room 5 contains 1d3+1 minor Xorn, but I don't see why elemental beings capable of phasing through stone would hang around a place like this, so instead, Room 5 contains a fountain flowing with smooth mud. Covering yourself in Mud gives solid protection against Lightningbees, and Carroll sometimes tasks his slaves to try to retrieve lightninghoney for him this way, though they also have to get past the angry Cu Sidhe and avoid getting eaten by the Annis. Stone-To-Mud can be learned if you are mad enough to drink the mud, or if you fully submerge yourself so that the spirit of the fountain can impart mud secrets to you. The Annis of room 6 is sometimes here, posing as a 'Mud Nymph.'
Finally, room 6 is the laboratory of an Annis. Big books of cannibal recipes and tomes of herbalism hang here, along with a magic mushroom farm and a collection of bones and a big cauldron full of Lightning Honey. The Annis is loyal to the Underking and to Carrol, and she gets to eat everyone Carroll sacrifices. She has a key to K3 and in addition to her monstrous claws, she has the following magic abilities that she could teach as spells if coerced.
- Alter Self- Though her true form is a giant hag, she will take the time to pose as an attractive 'Mud Nymph' in an attempt to separate and confuse the party.
- Choking Smoke Potion- Throwing the correct mushroom into her cauldron will fill the entire area with choking, blinding smoke. You have to make a con save to speak or cast spells, and visibility drops to 10 feet. This also will put the lightningbees to sleep, which is the main reason she developed this trick. It requires a mushroom, honey, and extra magical energy, which lightninghoney helpfully provides.
- Stone To Mud- She's learned this from her mud baths, though she doesn't want to compromise the stability of her lair.
- Man to Swine- A curse that requires the target to be naked, muddy, and human, she can probably only cast this on someone who falls for the 'mud nymph' routine or if she remains undiscovered and the players decide to get muddy to try to retrieve honey.
Rooms 1 and 2 are devoid of immediate threats, but 2 has a statue of a half dragon 4th level fighter, who was one of the encounters but was petrified when he looked down the hallway to 8, which has since been blocked off by some fluttering silk veils. Servants are meant to march through 2 blind and not listen to anything they hear, and anyone passing through WILL hear the medusa in 8 trying to get people to look her way and turn to stone.
Room 3- kept to scare new slaves into obedience, this room contains, not a Gauth, but a giant eyeball set into the floor. Normally protected by stony eyelids, it opens to glare at intruders, afflicting them with a sense of helpless terror and dread unless a save is made. NPCs will do anything to avoid nearing the eyeball. Rather than dictate PC behavior though (I've used fear effects in a past game and I wasn't really a fan), the effect on PCs is that their HP is immediately reduced to 1 as their limbs grow weak and trembly with fear, easy prey if they are forced into combat. The eye grows more and more bloodshot and shakes horribly if they draw near, trying to scare them off, but has no other abilities and can be easily slain if a player overcomes their fear of engaging with 1HP and moves in for the kill. If attacked at range, the eye will shut behind its stony lid, becoming immune to anything but dedicated melee smashing with a hammer or pick, or truly catastrophic damage.
Room 4 is ostensibly an alternate route to avoid the Ettin in 5, but the presence of a digester and a babau (an acid monster and an acid demon assassin) makes me think there's a Streetsweeper Ooze in here that has lost its alchemical behavior protocols and now is just a ravenous acid monster. It is pressed to the other side of the door as it hungrily senses the nearby servants, and will burst out in a surprise round if the door is unlocked, possibly blocking off escape if the players went down the hallway first. The door it is behind is very dusty and clearly hasn't been used in ages, while the 'dead end' is perfectly clear of dust, which hopefully will clue the players in.
Room 5 has an ettin with a collar and chain leading back into 12 and 14. There is also a winch mechanism that raises the secret door to the hallway south, but the Ettin will make operating that very difficult as it wants to smash people to pulp. If bothered by ranged attacks from a safe distance, the ettin will retreat to 12, crawling through the hall, and lay in wait around the bend in the hallway.
Room 6 has a door inscribed with symbols of some good god, and holes in the wall to 7. Access to this door is sealed off by spiked and golden chains from a Chain Devil. This vile creature is a servant of the God of Bloody Gold and has been tasked with preventing access to the shrine beyond. It gets 4 attacks per round and absorbs gold pieces=to damage dealt x10 on hits. It will wish to flay any players who do not worship the God of Bloody Gold, but will offer to spare them if they gift it 100xHP gold. Its chains are long enough to attack people in 7, snaking through the holes in the wall.
Room 7 is empty save for the holes in the wall and a skeletal adventurer. People looking through might lose an eye to the chain devil.
Room A2 has d4+1 shadows in it, and a giant mirror that is a portal to the plane of shadow and reflects nothing but blackness. The shadows will take over the shadow of anyone reduced to 0 STR by them, and from then on they'll be vaguely possessed. Whenever the shadow wants to do something, they have to make a save or act under the control of the shadow for 1 round.
The portal is open only when the mirror is uncovered and illuminated- in darkness, it can't reflect anything and shadows don't exist (this goes for the monster shadows too). The current shadows showed up when the Undermen retrieval squad accidentally opened the shadow gate, and the shades hope to catch the undermen when they return.
Room 8 Has a few more statues of various slaves and adventurers who were lured here or sought knowledge. Hanging from some chains is, rather than 1d3+1 cockatrices, an oracular medusa with a huge snake tail for legs that allows her to thrash and constrict the life out of anything that comes into the room, though another nasty trick she has is to haul constricted people up to her face so she can petrify them, and she'll bite off their eyelids if they think shutting their eyes will save them. She can only lie to people who can't see her, but must tell the truth to people who look at her. Anyone seeing her face will begin turning to stone from the feet up, and have 3 rounds/3 questions to ask her before their head turns to rock.
The mud bath in A1-5 will restore petrified people, and that's where the medusa will direct people who get a truthful answer on that subject. She'll direct people to come get the key from a broken servant statue in the room and enter K1 if obliged to give a false answer on the subject. She is very tired of being an immortal oracular monster and though she'll defend herself, she'd rather just die than spend her time locked up.
Room 9 is a a cluttered, abandoned storage room full of broken pottery, metal scraps and stone tailings, and moldy rags. It's a good place to hide people and things, but light here may alert the shadows in A2. There are dormant Itava parasites in some of the larger clay pots, as the encounter chart called for acid-spraying bombadier beetles and acid-spraying already introduced lifeforms seems close enough.
Room 10 is large, but empty, and there is a tablet on the floor by the north exit saying 'wrong way.' This is for the benefit of Undermen from K3, not the players.
Room 11 has a large gong. Beating it at the appropriate tempo alerts S3 servants to retract the ettin chain to presumably make the way clear for incoming Undermen troops.
Room 12 and 14 are unremarkable save for the long chain that heads through the south wall, and possibly the Ettin lurking within. It prefers room 5 though.
Room 13 has 2 gargoyles guarding the door. They will unlock the door for people wearing Undermen guard outfits, but will fight anyone else to the death.
Finally, the 'K' vaults.
K3 is a grand staircase spiraling down to level 7 (which can also be reached by the secret door in the level 4 ravine, or perhaps by cavern exploration around the Cathedral of Bloody Gold). Skullmoth lanterns line the walls.
K2 is an abandoned shrine with a dying god. This is supposed to be some angelic being that's like an elf that can turn into a whirlwind, a Bralani, which is as good a starting point as any. This god was an enemy of the God of Bloody Gold, but was cast down and now languishes in this forgotten shrine, wasting away without any worshippers. The god appears as a blue elf with white hair fluttering in a tiny breeze, emaciated and starved. They are the god of Underground Weather, and oversee things like cavern humidity, bad air, draftiness, and the occasional fog. They also are good and seek to punish the wicked, which is how they got on the bad side of the God of Bloody Gold. For this god to return to the celestial realm, the nearby altars of the God of Bloody Gold (the cathedral one, Carroll's dinner-altar, and the main one on the level below) must all be destroyed.
Miracles Granted to worshippers
Find The Draft- Divine knowledge of air currents allows the petitioner to find routes to surface airflow.
Air Pressure- Gusts of air and pressure differentials won't slam doors or extinguish lanterns for the faithful, though they may for the enemies of the faith.
Canary Guide- The ghost of a canary will be dispatched to warn the faithful of hypoxic environments and poison gas, and if possible, it will beat its wings to clear the air. Alternately, the faithless could be smote with a bad air pocket
K1 should contain a 5-headed cryo or pyrohydra, but instead it contains an Itava Hydra- 5 acid-spewing parasites that have totally taken over the body of someone- the torso is just an excuse as the head and limbs have atrophied and the parasites took over their functionality. A single hit cuts off a parasite, but it only DIES if 6+ damage is dealt.
If all the parasites are removed, the still living host is a '5th level lizardfolk druid' but actually it's a 6th level underman master corpsetaker who gained a sort of immortality via symbiosis with these parasites. He was the old mayor of the town and wants vengeance against Carroll, who locked him up here. He also will wax eloquent on the lifecycle of the skullmoth and how amazing their contradictory nature as biological being and negative-energy monster is, and generally come off as a creepy parasite cultist because that's what he is.