This is basically my take on the same tropes: megadungeon, At the Mountain's of Madness, sorcerer-king flavored superscience, expeditionary play, and a dollop of psychological horror.
And while the Obsidian City has been on the map, for a long time, it's only ever existed as a pile of ideas. None of my players have ever wanted to go there. (Despite the fact that this is where the crystal sword from the Meal of Oshregaal actually leads, and I've run that thing a few times.
I thought I would start by talking about the different factions of the Obsidian City.
They sealed their houses against the apocalypse, but it crept in just the same. Not the violence they feared, but slow starvation and creeping cold. Nearly all of them died in their houses.
And so the city is a massive maze of sealed houses. (I'll have to write a random generator.)
Most are sealed tightly. A few have been cracked open by looters.
Most are trapped.
Some contain edible foodstuffs: families of corpses, mummified by the cold.
Some contain undead: those same families of ice mummies.
Very few contain anything valuable, unless you are searching for shelter and a frozen corpse to chew on.
The Giant Penguins
They cluster around the harbor. They're huge, sluggish, and subtly deformed.
They aren't really a faction, but they're here because I read At the Mountains of Madness. They won't bother the party unless they enter the shipyard, because that's where they build their nests.
A smart party will rapidly learn that wherever there are giant penguins, there are giant sea lions. (HD 8, surprise 5-in-6). A party venturing onto the (half-collapsed) docks will likely have to deal with at least one of them.
This is you. You're part of an expedition financed by the imperial ambitions of Noth. You are a company of explorers. You are camped about a mile outside the city walls, in a makeshift harbor. It is fairly safe here.
There are other companies of explorers. These are your friends and rivals.
Korbith Mantukkar. Eight brothers. One sister. At least two dozen sled dogs. They spend half of their time fishing and eating. The other half, they spend being exceptionally competent.
Yesiderata the Mouse. Not a wizard, although many think that she is. Financed by Grimagangus, an actual wizard back in Noth, of immense power and wealth. Leads a company of mercenaries; one of which is rumored to be an orc. Interested in magic and nothing else.
Steerpike and Yattis. Historians, nerds, and miners. They don't belong here. This is their second expedition. Their first expedition ended with half of the party dead and/or missing. They're fools. They're cursed. They pay double and it's not enough--no one wants to touch them.
The Black Hats. Actually members of an Imperial prison cult, they chose to be assigned to the Obsidian city rather than the work gangs of more temperate climates. This is their prison sentence, and they have been sentenced to explore the Obsidian City until they die (or until the Emperor finds a better use for them). Murderers, rapists, atheists.
But let's talk about you.
You will probably be required to take an Imperial functionary with you at all times. The functionary's job is to make sure that you are acting in the best interests of the empire, that you don't damage anything too priceless (either artistic or scientific), and that you report back in a timely manner. The functionary also has a secret job as well: grab anything that might be a magical weapon, and keep it out of the hands of anyone who might abuse it. (A category which definitely includes the PCs.)
The functionary knows a lot more than she is allowed to tell you, in the vein of Weyland-Yutani.
It is Very Bad Form if your functionary dies or goes missing. It may even be grounds for exile or execution.
A word about exile: the storms only clear for about 4 months out of the year. And so the Nothic ships all sail up at the start of the season, map out as much of the city as they can within the time allowed, and then sail back at the end. For safety reasons, the ships all sail as a fleet.
Therefore, if you allow your functionary to die, you may be thrown in the brig until the end of the season, when you will sail back to Noth for trial.
When Noth first began exploring the Obsidian City, they noticed that on some days, a thick fog would creep in and a number of unusual things would happen among the crews and parties.
Fear, paranoia. Mutiny was muttered. A few stabbings occurred. And there were hallucinations, too, because how else do you explain the man who got lost following his own mother into the mist? Or the enormous serpent, a mile long and twenty feet thick, that multiple people saw slithering among the domes?
The players don't know it yet, but yes, those things are hallucinations.
Fifty years ago, Noth waged a war against the frost/storm giants (same thing) and eventually won. They razed their towns, sank their ice-berg ships, and occupied their (surprisingly humble) capitol city. They were called the Stormlords, and they are mostly all dead.
Digression: The Stormlords are not a subspecies of giant. They're just a bunch of normal giants who lived in a snowy place. They are not immune to cold or lightning. They wore big bundles of fur like normal people, and just happened to be good at controlling the weather, because you have to be to live that far north.
And yet a few Stormlords yet remain. Some were enslaved by Noth (and in fact, there is one here in camp, in charge of building walls and moving heavy cargo). Others dispersed to other icy little islands, where they are mostly starving. And one iceberg ship survived, and its crew yearns for retribution against the Empire.
This one remaining iceberg ship is here, moored a mere half-mile offshore from the Obsidian city. The explorers have sailed past it a dozen times, but have never noticed it, because it's always been hidden by the mist.
The giants have two goals here. First, they want to cause as much damage to the explorers as they can without being noticed. Second, they are searching the ruins for anything that might help them reclaim their homeland, or forge a new one.
And so the maddening mists are the first step in their plan. They use it to cause trouble, like when Josafek was torn apart by his own sled dogs, who mistook him for a bear. They use the mists to obscure their own expeditions into the city. And they use the mists to pluck information from the minds of the explorers (such as their greatest fear).
They have one last tool: an enormous whale named Yonakkuk that they have been bargaining with. They have given Yonakkuk food and magical protections. In return, Yonakkuk has attacked the expedition, sank one of the ships, and eaten a dozen men. (Another reason why the ships don't sail alone.) But he is a slothful and capricious ally.
There are not many giants left. They are dying, and the ones who are not dying are succumbing to despair. More than one of them has jumped into Yonakkuk's mouth.
These were the servitors of the city's old inhabitants. They were born from the flesh-engines beneath the city.
They have worked to reclaim the broken areas and make them habitable again. There are a number of subterranean agricultural zones that were once destroyed, but the Vatborn have returned them to working order.
This hasn't been an easy process. They've gone through several eras of high populations followed by die-offs, but each time, the surviving flesh-engines resurrect their race.
Each strain was created for a specific niche, and each one now forced to fill all the niches of their society.
The Swodinnar are muscle-freaks. Anabolic warmachines worshipping a cult of strength. Massively overdeveloped hands and forearms. Only seven feet tall but as strong as an ogre. They are searching for their ancestral armors and weapons. They occupy Green Eden, and are the most peaceful of the vatborn.
Their previous leader was Warlord Shudrok, who lead them into military success and societal failure. He's currently imprisoned. The current leader, Speyjin, is Shudrok's polar opposite. (At least, by the standards of Swoddinar society. By our standards, he's still an aggressive muscle-freak).
Like most of the clone races, they cut the faces of their babies in order to give them distinguishable scars.
The Blue Eyes are a clone race of small, bearded men. They have an enhanced pain response, and flinch from loud noises.
They were created as an act of revenge. Once, a small bearded man inflicted some great humiliation on some great fleshcrafter, and thereafter the fleshcrafter wanted the joy of torturing and killing that man again and again.
The Blue Eyes are thieves and assassins. They live in the Blue Eden, where they torture on stages and investigate the secrets of the flesh-engines.
The Red Eyes were the sex clones. Their skin is a lurid pink, and their hair is a pinkish white. They live beneath the dome of the Red Eden where they sometimes trade with the members of Black Eden.
The Red Eyes are the most advanced botanists beneath the Obsidian City. Without their innovations, all four of the surviving edens would almost certainly decay. They create a number of exotic plants.
The Golgurrians are clones of a powerful sorceress of the same name. They are arrogant and combative with everyone except the Red Eyes, with whom they trade. They have blue skin and all wear an eyepatch. They know the secret to creating moculi, and each Golgurrian usually has one of the little things flying behind their head at all times. They look like yellow-orange flying eyeballs dragging a few loose tendrils, each the size of a baseball.
HD 0 HP 1 AC plate Atk none
Fly 12 Int 5 Mor 2
Stare - 1 damage per turn, no save. If enough moculi stare at an object long enough, it will break or fall over. For large things, this might take a very long time, and a great many moculi.
Some of the Golgurrians are trying to ressurect their predecessor. They have been tampring with their flesh-engine, and some of the youngest generation are now being born with partial memories of the city above, which is exactly what they want.
If their peers discovered this experimentation, these Golgurrians would certainly be killed. Tampering with the flesh-engine is the deepest taboo. If it breaks, it means the end of their species. Or worse, they could share the fate of White Eden.
White Eden is a maze of darkened halls and frozen, subterranean forests. Its flesh-engine is cold and grey, and only spawns abominations. Ghoul things, born dead, each pregnant with generations of toothy monstrosities. The vatborn have been very careful to seal off the area around White Eden, but the players will almost certainly open it up.
As the players begin exploring the city, they can't help but notice the gleaming tower at the city's heart. It's forty stories tall and holds what looks to be an enormous sphere at the top.
They won't make it to the spire. That's late in the campaign. The spire is where the Ool live, and they will not allow anyone to approach. The door is guarded by a quartet of flab men.
The Ool are flying mechanical octopi. Each with four rubbery tentacles and a brain encased in a hemisphere of crystal. They are mind-flayer analogues.
They consider themselves the masters of the city. Above ground, this claim is rarely challenged.
They have a pair of flesh-engines (or more) in the tower, and they create their own vatborn slaves. These are the flab men and the thin women.
They are never encountered without some of their mind slaves, which may include any of the other factions in the Obsidian city (except the Labyrinthine).
HD 8 AC chain 4xTentacles 1d6 or grab
Fly 9 Int 16 Mor 7
At-Will Spells - telepathy, read mind
Daily Spells - dominate, mind blast
Tentacles - Each tentacle has 1/4 of the Ool's total HP. When an ool loses 1/4 of its total HP, it loses a tentacle. Grabbed opponents can be thrown (on a hit, both take 2d6 damage) or squeezed (automatic 1d6 damage).
Braincase - The crystal hemisphere covering their brain can be attacked directly. It has AC plate+2 and HP 6. Damage to the braincase doesn't affect the Ool's normal HP total. If the braincase is shattered, the Ool dies instantly and dramatically. Ools never let their braincase get within melee range (their tentacles are long enough).
Mindslaves - Each Ool is accompanied by 2d10 mindslaved minions. If the Ool dies, the minions are instantly freed from its domination.
HD 4 AC leather Punch 1d8
Move 12 Int 6 Mor 12
Regenerate - Full HP at the start of each of each turn. It cannot recover if brought to 0 HP, but the corpse will grow interesting tissues and fans of blood vessels while it lies on the floor.
At-Will Spells - levitate (self only)
HD 4 AC leather Claw 1d8
Move 12 Int 6 Mor 12
Regenerate 2 HP per Round.
Frenzy - Attack twice per round if they are at full HP.
At-Will Spells - levitate (self only)
*Both flab men and thin women use their levitate ability to navigate the many vertical access tunnels beneath the Obsidian City. It also helps them ascend the Spire of the Ool.
Centuries of constructive instincts, layered together in a multinucleate matrix. Ooze retrained to be an architect.
The fastest way to explain it is to imagine an enormous black pudding, impregnated with the urge to construct, repair, and correct. They filled it with the minds of millions of hermit crabs (the closest they could find to an architectural instinct in an animal brain) and hundreds of brains of actual architects. Then they spent another century training it.
Now it maintains the city.
It struggles on the surface, where the cold freezes it and it becomes the "obsidian" of the Obsidian City. But in the warmer tunnels, it has full mobility and awareness.
It doesn't mind the vatborn as long as they don't damage the architecture.
It uses secret hydraulics to pump itself around, moving from secret reservoir to reservoir. Vast cisterns of the stuff. Don't break the pipes.
It just wants to maintain the city. The immobile parts--it doesn't care about the people or the objects. You might be able to talk to it. Some people already do (but they probably aren't people you want to talk to). In terms of raw power, it's the elephant in the room. Six thousand tons of subterranean hydraulics can throw a cow across ten city blocks.
It's limited by the cold, but there's a way to turn the Black City into a bubble of tropical climate. (See also: the Goals of the Red Eyes.) (See also: the Climate Control Center.)
It's an ooze, but it's also a mindset, but it's also a disease.
Sometimes explorers catch it. You might find one of the members of Steerpike's party, rebuilding a crumbling wall with frostbitten fingers and glassy eyes.
Deeper down, you might find a man trying to build himself into the wall. Wedged into a gouge, stacking bricks up in front of his shins. Using blood and shit for mortar. Cutting out pieces of his legs so that they'll fit better. (The cold has made his legs numb. He doesn't feel any pain. Otherwise why would he be smiling?)
And even deeper down, you might find the same thing, except this time, the wall is helping.
Stats as a black pudding with Int 10 and infinite HP. Dealing 50 damage to it will usually drive it back, unless it is protecting something specific.