Procedural Dungeons with Weighted Dice
How Fibonacci sequencing and bell curves create organic dungeon crawls
…and welcome back everyone. I hope you are doing well. I’ve been busy as usual. Everything is bigger than expected in the garden, the grasshoppers are back and luckily aren’t that smart. So I’ve blocked off large portions of my garden, and so far they can’t figure it out, and the bee’s still have access.
Looks a little like a makeshift dungeon…
Speaking of… How about we design a hybrid Fibonacci/bell curve dungeon generator that builds rooms, encounters, and details on the fly?
First we need some foundations to build on. Last time, towards the end, I talked about using a combination of Weighted and Bell Curve rolls; the first providing the entryway and the purpose, the latter, let say fills the room with furniture and other details. We'll also want to fill our dungeon with some encounters, treasures, and a few scattered traps and other hazards. The likes of which should of course match your theme. I've decided that I'm not really in the mood for anything in particular, so I'll roll off screen later; in case I change my mind.
So we need the following:
Weighted d12 (Fibonacci) for room types (common/rare)
2d12 bell curve for room details (shapes, hazards, treasures)
Nested tables for dynamic encounters
You could easily fill in the tables with shop locations, houses, local shrines, for example if you wanted to curate a town. Keep in mind that curation is the function of our tables. We are presenting a story, we're just not entirely in control of how it's told. As a solitaire player I find these types of curated tables to be invaluable. You can take a few simple ideas and apply them some a number of tables, and then overlay some rules on top. What you end up with is a unwritten story that you then get to explore and expand on. Today we're making dungeons, but you could easily modify these tables to fit any type of experience you want to play through. Adventures of all sorts, from dungeon crawling to space dramas.
Before we go to space, let's finish working on this dungeon generator first.
ROOM TYPE TABLE (Weighted d12)
Let's start at the top by creating a Room Type Table. This table determines the fundamental nature of each dungeon room by giving it a foundation. The Fibonacci weighting (4-3-3-3-2-2-2) creates a natural distribution where common spaces appear frequently while special rooms, such as shrines and boss areas remain rare. In this case, subtract -1 to all dice rolls would prevent a boss/shrine roll until certain conditions exist; for example, you've found a way to kill them. Or let's say you find a special item, which also happens to give you a +3 or more to all rolls, thereby creating a curve in the weighted roll that pushes you closer to a boss encounter.
To use roll 1d12 and consult the table. The "Boss/Shrine" category(11-12) combines climactic battle spaces with eldritch horror rooms, giving them equal but rare weight. This maintains tension - players know when they roll 11+, something significant awaits.
COMMON ONE (Weight: 4 - High Frequency)
Roll 2d12 for subtype (bell curve distribution)
These rooms appear often (about 31% of the time) and keep the dungeon feeling grounded. The bell curve on the 2d12 roll means most results cluster in the middle, creating predictable patterns with occasional surprises.
Bathing Area (2-4, ~8% chance): Rare but memorable. That ‘safe to drink?’ roll (50/50) forces interesting choices—do you risk dysentery or push on thirsty? Of course you can adjust this to your liking, tune it to fit the experience you expect to have. Just like adjusting your attitude changes the way you experience life.
Empty Room (5-12, ~44% chance): The workhorse of our dungeon. Nearly half of Common One rolls land here. The 20% hidden switch tease rewards without slowing things down. Empty Rooms... I know, how boring. But honestly quite common and expected. By 'empty' I'm simply implying a standard boring room with nothing special going on. It might have a bench or two, or a chair, a fireplace, and other details, however, for the most part, there's nothing of any importance really... Unless of course there's a hidden room...
Storage (13-16, ~24% chance): Common enough to feel real, but not spammy. The 1d4-1 items ensure players always find something—even if it’s just rope.
Short Hall (17-20, ~19% chance): The 25% draft chance and turn mechanics make corridors feel alive. Poke at all the walls just in case!
Guard Post (20-24, ~8% chance): As rare as the Bathing Area, but for lore instead of survival. That 20% journal chance means they’ll find 1 in 5—enough to tease, not spoil.
COMMON TWO (Weight: 3 - Moderate Frequency)
Roll 2d12 for subtype
These rooms appear less often (~23% of total rolls), adding variety without overwhelming. The bell curve still applies, but the subtypes are more dramatic.
Dining Space (2-4, ~8% chance): A gross treat. The 1-in-4 edible food is a cruel joke—perfect for horror. Who left that fresh bread? (Spoiler: Don’t eat it.)
Long Hall (5-9, ~30% chance): Our most frequent Common Two results. The ‘sense of being watched’ plus branching paths creates tension. That 25% hidden path means you'll second-guess every corner.
Collapsed/Empty Room (10-16, ~44% chance): The hybrid d4 roll splits this neatly, half collapsed (danger), half empty (mystery). The 20% hidden switch mirrors Common One’s Empty Room, creating consistency.
Barracks (17-19, ~14% chance): Uncommon but impactful. That 20% armor chance (AC+1) makes it a mini-treasure room, so scour every bunk.
Cistern (20-24, ~8% chance): The rarest Common Two space. Hiding loot here (DC 12 Perception) rewards for observant characters. Perfect for stashing a key to the Boss/Shrine!
ROOM SUBTYPE TABLES (2d12 Bell Curve)
Each room type has its own subtype table that adds depth and variation. The bell curve distribution (2d12) means most results will cluster in the middle ranges (7-19), creating predictable patterns with occasional extremes.
For example, when you roll a Common Hall (1-3), then roll 2d12:
2-5 (8.3% chance): A cramped closet
6-8 (16.2% chance): A narrow corridor
9-12 (27.8% chance): An antechamber
13-16 (27.8% chance): A meeting hall
17-24 (19.4% chance): A grand gallery
The size and encounter modifiers naturally scale with the rarity of the subtype. The Encounter Modifier (Enc. Mod) is a percentage-based adjustment that influences two key aspects of your dungeon encounters. First, Encounter Likelihood. A modifier to the base chance of an encounter occurring in that room.
A +50% Enc. Mod means encounters are 50% more likely than the dungeon's baseline. Negative values (-30%) reduce encounter frequency creating a safer space, maybe a good place to camp, heal, cook some food if you have any. Second, is Encounter Severity, which scales the difficulty/quantity of enemies or hazards. For example, a +100% Arcane Vortex suggests deadlier foes (more elites/uniques) or added environmental threats. We'll get more into this topic in a moment when we start working on the Encounter System. For now, place it someplace in the back of your mind, someplace within reach. If you have too, knock off some of that crap that no longer serves you so you can make room for something better.
Promise, it's worth taking a look at what's going on in the back of the mind. Each of us develop filters, often based on past experiences, that we see through whenever we look out into the world. Everything we do and experience is then tinted by these filters, just like the Encounter Modifications to each room. Each experience we have is filtered with a certain level of expectancy. Some rooms, you can tell right off that there's gonna be something going down in here because you know from previous experiences what to expect.
Of course adventurers such as yourself know better, right? We know that far too often, what we expect is what we see due to our expectancy of it. In otherwards, we often find what we're looking for just because we expect it to be there; even if it's not, we twist the world to fit our personal truths, real or not. And that's why taking a look back there is important. Lots of those filters can be cleaned out and or readjusted. Often they are useless, but we hold onto them nevertheless and never question why.
Open all the windows, get some fresh air in there, and question everything, always.
SUBTYPE TABLES (2d12 Bell Curve)
Again, keep in mind, the Mods for Encounters because they modify the Encounter System later. I crunch traps and encounters together here, you of course are free, and encouraged to come up with your own trap mechanic using all these cool mechanics we've been going over. Further, injected about the tables are keywords, meant to be used as seeds for your own creative experiences.
Note the bell curve distribution clusters results toward the center, making extremes less common.
Alchemy Lab
Forgotten Library
Prison Cells
Eldritch Shrine
DYNAMIC HISTORY (ROOM LORE) (Optional)
This optional layer adds narrative texture to rooms. Roll 2d12 when first entering a room to determine its backstory:
Common results (7-19): Suggest typical dungeon features
Rare results (20-24): Reveal dramatic story elements.
These histories modify encounters, making them more than just combat - a Murder Scene might spawn vengeful spirits, while an Abandoned Shrine could provide blessings. The magic is in your imagination. Roll when first entering any room to uncover its past. Here I am curating the experience to fit a particular mood, obviously, so imagine this filled with your own experience.
ROOM CONNECTIONS AND EXITS
NUMBER OF EXITS (Roll 2d12)
We need to exit the rooms right? This determines how many paths lead out of the room, even if that's simply the way you came in here (2-5). Here we are using a bell curve where you'll find most results have 2,3 exits per room. You could apply an environmental modification if, say you wanted a more chaotic layout, with lots of exists by either adjusting the table itself, or applying a modification to the roll itself. A +5, or even a +10, even if a large value seems awkward, who cares?
It's not like anyone's looking, or supposed to be looking (everyone turns and stares right at you...) The character down there in the dungeon has no idea that you're up here rolling loaded dice to determine its fate. And if you're directly involving your 'self' down there and taking things a bit more serious, then you're not supposed to know that you're rolling a loaded 2d12 either. So pretend you don't know and carry on as usual. It's good practice to let things be what they are. Practice in game, then take a chance and practice outside the dungeon. Just like your characters, there's no place to go, nothing to hang onto. Just be and have fun before you're someone else again.
Bell Curve Note:
~44% chance of 3 exits (balanced exploration)
~8% chance of 1d4 exits (unpredictability)
2. EXIT TYPES (Optional, Roll per Exit, 2d12)
Determines the condition of each exit.
TREASURE TABLES
When you find a treasure room, Roll d12 for tier (weighted 5-3-2-2). Then, Roll 2d12 on the appropriate table. The system ensures common items appear frequently while powerful artifacts remain rare. Notice how, common loot (5/12 chance) includes practical supplies and unique items (2/12 chance) can change game play. Here again, is another good example of modifying rolls so that certain values are not possible unless certain conditions have been met.
The condition doesn't have to be listed anywhere on any table. It only has to be in place on the table instructions themselves:
Special Table Rules: Subtract -2 from all rolls above 10 unless you've been told to ignore this rule. Or just have them roll a d10 and skip the awkward instructions. I'm all about awkwardness, so...
Roll only if room type permits (Treasure Vault)
Loot Tier (Weighted d12)
Total Weight: 5 (Common) + 3 (Uncommon) + 2 (Rare) + 2 (Unique) = 12 Chance Formula:
Common: (5/12) × 100 = 41.7%
Uncommon: (3/12) × 100 = 25.0%
Rare/Unique: (2/12) × 100 = 16.7% each
Specific Items (2d12 Bell Curve)
Roll per tier
Encounter System: "What’s in This Room?" Cheat Sheet
Let’s make this crystal clear, no math, no confusion. Just match your Enc. Mod to the table, apply the Needed Mods, and roll.
Step 1: Room Tells You What to Expect
Every room has:
Enc. Mod (how dangerous it is)
Needed Mods for Encounters (what kinds of threats appear)
Example: Active Ritual (Eldritch Shrine)
Enc. Mod: +100% ("Oh Crap" tier)
Needed Mods: +3 Elite, +2 Unique (cultists, monsters)
Step 2: Encounter Matrix (Pick or Roll)
Step 3: Apply "Needed Mods"
Your room subtype (Active Ritual, Warden’s Lair, etc.) overrides the default biases:
Active Ritual: Forces Elites/Uniques (even if Enc. Mod is low).
Warden’s Lair: Always has +4 Elite (boss fight).
And that’s what I’ve got for you this week. I hope you find it useful and enjoyable. Personally, I’m getting a bit burned out the topic, and I think it’s time we started to switch things up and see what else there is to explore. Or maybe the dopamine hasn’t run dry quite yet. We’ll see what happens…
In the mean time,
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The land is plagued by bizarre phenomena: radiation zones, physics anomalies, electrical surges, and gravity distortions, while its mutated wildlife and deranged human survivors pose constant threats.
Designed for low-prep solo or group play, the game uses minimalist rules, with added systems for crafting, repairs, and conditions adapted to this harsh survival-horror setting.
I’ve been playing it for sometime now, and I am working on a review/playthrough soon. So keep an eye out for it, and be sure to get a copy of your own.
And like that we’ve reached the end. I hope everyone is doing amazing, and if not, keep pushing forward with me because I can’t do it alone. Hope to you see all next week, please take care of yourselves and one another, we’re all we’ve got.
This is great! And thanks so much for the shout out!