Sunday, May 5, 2024

The Infinite Orrery

One of the things I like about blogging is that it exorcises the daemons and geniuses that haunt your head. That is, it gets your ideas out of your head and onto paper so they stop bothering you. Maybe other people pick them up, maybe not. 

Here's a capsule game idea that I had that I don't have time to make right now. It's called The Infinite Orrery. The premise is that the players are apprentices (querents) serving a wizard. The vibes are Witch Hat Atelier, Wizard of Earthsea, Kiki's Delivery Service, and Howl's Moving Castle. 

Click these to make them big mode.

The gimmick of the game is that you play as a wizard's apprentice, called a querent. All the players live in the wizard's house, doing magical chores, gathering reagents, studying astronomy, creating potions, and learning spells (more on that in a bit). There's no limit on how much magic you can cast, just the outcome of what happens as you meddle with magical dangers. The GM takes on the role as the wizard, sending the players on quests. Eventually, players would graduate from their apprenticeship, and sail to another island to take on the challenges of the archmage and graduate to wizard-hood.

The game book is written to be as diegetic and in-character as possible. There are no numbers. You create your querent by doing a tarot reading and making decisions about important choices in your life. As you make choices, you gain defining virtues for your character - attributes, lores, items, etc. You start with one spell, too, based on your zodiac sign.

It was a challenge to write the resolution mechanic in-character, but I think it turned out oh-kay. There's some missing details here, but I only wanted to spend one spread on any concept. Basically, the more advantages you have, the more cards you draw. (An idea I briefly considered was using a magic 8 ball as a randomizer instead.)

The basic gameplay flows between two phases - the quest stage, when you're adventuring and solving open-ended problems on your island home for your master, for your neighbors, or for yourself - and the study stage, when you learn magic. 

I wanted to give the experience of learning magic both in and out of character. Players have to memorize the "Sidereal tongue" which is represented by a runic cipher. 

The players do not get to read the list of spells. All spells are 3 letter words (AGE, AID, HIT, HIP, HOT, FLY, FLO, FLU, etc.). During the study phase, you guess a combination and write down the runes of the spell you're attempting to learn. If you guess one of the spell words and your runes are correct, the GM (the master of magic) shows you the spell page which you copy down into your spell book. As you play the game, you literally make a little spell book out of a journal, with runes and illustrations on one side and in-character game rules on the other.

The master of magic has a separate book from the players. Some of it is spells (which they reveal to players when they correctly guess the letter combinations) and some are quests. Basically, every section of the island has open-ended problems that need to be solved, and different items that the master of magic might ask his apprentices to get for him. The GM just needs to choose a few for an evening of play, and see what sort of trouble the querents get into. The intention would be to include a lot of randomizers, too, to help drive play and keep things interesting.

Anyway! This was just a little thing I put together as a diversion. I don't have time to take on another project at the moment. I hope you thought it was interesting!


I made this proof of concept with a mishmash of borrowed assets. 

The whole project is spawned by an admiration for the feelies from video games - the Book of Patterns from Loom or the Codex of Ultimate Wisdom from Ultima.

Thursday, April 25, 2024

A Campaign Where There is One of Anything

It is well documented in the blogosphere that D&D fantasy produces a shift from fantastic as unique to fantastic as science. For example, in the myths, there is a single minotaur (literally, Minos's bull). The monstrous son of Minos, the King of Crete, is trapped in a labyrinth so that it could not escape and is fed captive prisoners. In D&D, a minotaur is a species of bull-person who, bizarrely, is good at navigating mazes.

Here is a way to frame your next campaign: There is only one of any fantastic thing. 

Create a map by filling it with unique things

Take your RPG book of choice. Take your copy of 5E Player's Handbook, Dungeon Master's Guide, and Monster Manual. Next, take a hex map. Then, fill each hex with one thing. (You can start with just three hexes, if you want.)

The old man in the forest can scry far off places. When you wish to gain the benefits of this awesome magic, you must first fulfill his bizarre wishes.

The passage through the mountain is blocked by the sphinx. She asks riddles of those who pass by, and slays those who cannot answer. Only the very wise may leave the valley.

The mountain itself is the haunt of the dragon. Its treasure is legendary. 

In its treasure horde, there is the flaming sword. It's said to be the sword placed by an angel to guard the Garden of Eden (but was later stolen by the devil). If you fight the dragon, you can gain the flaming sword.

Do the same with your character options

If there is a fighter, they are the Fighter. The prince that was promised. The eternal champion. 

If there is an elf, they are the elf. They are the King of Elfland's Daughter, brought into the world of men by the Prince of Erl. 

If there is a druid, they are the druid. The last in an ancient line of magical guardians, born of devilish parentage like Merlin. 

As adventures happen, place more unique things in far flung places

The fighter dies. Alas! However, the Holy Grail can give resurrection to one holy person per generation. Quest for it. If you find it, the fighter may be restored to life. The grail is taken by angels to heaven, to be filled with the dew of life for the next generation.

Because no ship made can get past the kraken, you must sail to the Isle of Winds where you must enter a joke contest with the god of winds in exchange for his flying carpet. Then, you can fly past the kraken to get to your destination. 

Even with unique fantastic elements in each hex, the world will feel magical. 

Further Reading

As ever, I'm not the first person to play in this space. Here is some further reading if this idea appeals to you. 

Monday, April 15, 2024

Interesting Social Situations, or The Discourse Post

The OSR space has discussed the importance of framing interesting combats. It has not done the same amount of work to provide concrete advice on running interesting social encounters. This post tries to talk about how to make social encounters more interesting and give more space for player choice.

This post contains two polarizing subjects in the RPG community: social mechanics and "systems matter." I swear it's not just discourse. I swear there is a point I am making here for the OSR blogosphere. If you engage in good faith, I think there's something here of value.

Social mechanics? Off topic!

"Nyehhhh!" I hear you saying. "I like [this system] that makes social encounters actually interesting! Instead rolling to swing a sword, you roll to make an argument..." That is not what this post is about. This post is not about social mechanic systems, but making interesting social encounters.

If you want a social mechanic system, here are two that I like: 
But--again--that is not what this post is about.

The Approach Layer

It might sound simple, but I think it is very important to articulate: making choices is what makes games "games." It's what is fun. It's what we like. When you run a game or design a game system, this is the thing you're facilitating.

When you are playing an RPG, you decide what your character does. All at once, you are interacting with many layers all at once.

My character knows more than me about combat. I know so little, I really had to struggle to think about what my body would be doing when I tried to throw someone. Do I think I could fight a level 1 rat? Absolutely not. I would definitely run away if a rat was attacking me. That's the "real world" layer.

The abstraction layer is the actual dice rolling part. The abstraction layer helps me make informed decisions. If I think the werewolf is too strong to be pushed, I won't try it. Beyond that, it's not what's interesting.

Yes, I care about rolling a d20 and the combat maneuver bonus and combat maneuver defense. Those things have texture that matters to me as a player. I like thinking about them. But it's not really what's interesting at the moment of play

The approach layer is where the good stuff is. This is where I solve the problems of the encounter. How do I deal with the fact that there is a werewolf in front of my character? If I can't hurt him, what do I do? Do I run away? Do I try to frighten it with my torch? Do I try to bullrush him off the cliff? This is the layer in which I am making choices. 

Let's try to adapt this metaphor for social encounters.

So, even though I know more about talking than I know about combat, I'm still not really interested in putting too much emphasis in exactly what is said. I might voice my character's statements exactly, like an actor. I might also just say, "I'm going to try and make an argument that if he helps us, it will please his ancestors."

I don't need to be poetic here. But I also can't just say "I try to convince the king." That's like saying "I'm going to defeat the werewolf." 

I'm going to defeat the throwing it off of the cliff. = I'm going to convince the making this particular type of argument

In summary, I really like this bit from the GLoG

> Roleplaying
You don't have to talk in a funny voice, but you do have to tell the DM what you are saying to the NPC, and how. 

It's not a division between player skill and character skill. If you don't understand that bootless goblins are susceptible to intimidation, and proud dwarven kings are not, you are bad at this game.

Common sense negotiations are one of the skills that this game tests.
This articulates exactly what I'm trying to get at here. It's not about saying exactly what your character would say. It's not about acting. It's not about being 'charismatic' in real life. It's about the approach.

(Arnold goes on to have some solid ideas about social encounters, here.)

Making Social Encounters Interesting

This is a pretty boring combat: 
You enter a 10' by 10' room. It is made of stone and empty, except for a skeleton. It attacks.

Similarly, this is a pretty boring social situation:

The knight refuses to give his name, and no, he's not wearing any heraldry. Just a featureless grey knight. He demands you give him taxes, or he'll attack.

Nameless NPCs, untethered to the setting or lore of the world, unable to be moved by any argument--a mere pretense for a shift to the combat phase. Boring.

Luckily, we know the essentials of how to make a satisfying OSR encounter: information, choice, impact. Let's talk through it for social situations.


Players can only make informed choices when they are given information. Because RPGs are limited in how players can get information - the GM has to tell them, verbally, what's going on - the GM should be generous with the information they present.

This means the GM should freely tell the players an NPC's emotional state. No "Insight" checks. Just tell the players what they think is happening behind the NPC's eyes.
  • The satrap is furious.
  • The witch queen is cautious, she seems to recognize the danger that you pose.
  • The guard is hinting at wanting a bribe.
  • The princess is being polite, but you see she is secretly sad.

What if the NPC wants to obscure information?

Competent NPCs can, of course, try to lie the players, wear disguises, hide their intentions, or otherwise deceive players. Like mentioning scorch marks that indicate a hidden trap or a slight draft that indicates a hidden door, you need to give players something to probe to receive hidden information.  

In these cases, you give the "landmark" information, and hint that there's something more at the "hidden" or "secret" level. 
  • The vizier is probably lying to you through his ingratiating grin, rotten teeth, and greasy beard--but the offer seems good. You wonder what his angle is.
  • At the mention of "the princess," the knight tenses up and gets a far away look in his eye.
  • The way the princess treats you at the ball is not at all in keeping with the rumors you've heard about her character. She's being vivacious, kind, talkative...not melancholy at all. 
The players will need to investigate - during this scene or during subsequent fact-finding missions - to learn more.

Speed up lengthy back-and-forths
In the same way you don't have to roleplay every word you say (if you don't want to), you don't have to actually have time-consuming conversations. If something stops being interesting, you can jump to interesting part. 
  • After hours of negotiation, the dwarf merchant gives his final offer.
  • The tea with Aunt Helga is tedious and uncomfortable, but at the end of it she hints that she'll update her will if you get married by the end of the year.


OSR problem solving maxim: There should be no single obvious solution, but many possible difficult solutions. 

Applying this to social situations, you have the essential problems of diplomacy. Fundamentally different world views at conflict with each other. Competition over limited resources. Powerful emotional forces like love, vengeance, and piety.
  • The humans wish the dwarves to help rebuild their war-torn city, but dwarves refuse to accept their payment of gold and jewels because Moradin gave them the rights to all fruits of the earth at the beginning of time. "They would pay us with our own robbed coin?"
  • The cleric pities the heretics suffering under the twin burdens of ignorance and disease, but will not use his healing magic on those who have not converted to the Faith. The proud people of the Vale will never give up their ancestral ways.
  • How can you ask us to come to parley with the Bloody King? He murdered our father.
Create situations rich in opportunity for player choice, creative problem solving, and unique approaches to understandable problems.

"Where are my kindred? / Where is the giver of treasure?"
Regardless of what social system you use, I reject the premise that you can roll a nat 20 on a Charisma check and make people act out of character. The primary way to convince people to do things they don't want to do is to give them something that makes it worthwhile. An essential part of social capital is capital. Tit for tat. 

Every NPC must be defined by their likes and dislikes, and their wants and needs. By giving the NPC something they want, the PCs can oblige them to make actions on their behalf. This can be gold, but it can also be certain actions. 
  • The duke wishes to secure his dynasty. Do you have magics that guarantee him a healthy, male heir? Can you guarantee his wife's fidelity?
  • The cardinal is sad for the by-gone days of the church's glory. If the lost Bell of St. Sadwick could be recovered, he would consider it a sign from God that your cause is just.
  • The guard would have taken your bribe last year, but the new captain is a real hard ass. He can't risk it now. But if there was a new captain, sure, he could help you on the night of the ball. And no, by Mythrys, I don't mean kill him! Just like, get him demoted or something.
GMs should have a good answer prepared for whenever the players ask: "What do you want?" Let the answers tie the game back to the gameplay you want in your games, like adventuring and dungeoneering.


Success or failure should change the situation and drive the game forward. "Nothing happens" is a boring result. Diplomacy is the art of compromise...and a good compromise leaves everyone mad. Staying true to the principles of "Information," the results of their diplomatic endeavors should be communicated completely and honestly.

Depending on how the players approached the problem, different NPCs and different factions should update their state and their relationship with the players. This can cause a new problem ("Well, we royally pissed off the dwarves, so they've all left the city...") or perhaps the players have achieved a brief respite. 

Friday, March 15, 2024

The Ludologic Plague: Practical Advice for Running Derro Encounters

To understand this post, I need you to go read this post from False Machine. Ready? OK, let's move on.

The Derros are characters with a history before D&D, and Patrick Stuart's interpretation of them in Veins of the Earth is my favorite iteration.

Whereas he relies on these wonderfully flavorful and evocative splorshes of text to give you a sense of the Derro, I am going to give you practical advice on using them in your games. 


The Derros understand that they live within a metatext. This is a terrifying existence for them. The second they are forgotten or unobserved, they do not exist. In the frenetic moments that they do exist, they put all their thought into escaping.

The Derros recognize that PCs are special. They torture various "named" creatures to see if they are PCs, but can quickly find out that they are not. They interrogate PCs to understand the "Machines" they use.

A machine is a game mechanic. 

The Derro lack the language to describe game mechanics, but understand some things as metaphors. A book of rules. Fate is random chance--sure, call it dice. These are all machines

They ask about the machines. Is the book brown? Is the book little and brown? Does it come in a white box? The one that is brown does not come in a white box? Do not lie. 

They address the "people behind your eyes." The people who "inhabit your skin."

They want to go back with you to the "upper world."

Every encounter with the Derro has this motif, has this refrain. 

Eventually, the PCs will begin to understand the gimmick the GM is trying to pull with the Derros. Then, they are infected.

The Ludologic Plague

Derros carry a magical disease called the ludologic plague. When PCs start wondering out loud, at the table, if the Derros understand that they are in a game--BAM! 

Every time a player says something about the Derro understanding the real world, the GM takes a post-it note and puts it inside the book. Put it anywhere. The game text hidden by the post-it note no longer exists. The GM can't see it, the players can't use it. The post-it note represent the ludologic plague spreading. 

If the players ever tell the Derros about the "machines." If they ever metagame and start talking, in character, about the mechanics of the game--the PCs become infected themselves. 

Put a post-it note over parts of the PC's character sheet. That part does not exist. That part cannot be read. 

The Derros have false machines that can control the plague. They can write on post-it notes that exist in the real world. They can rewrite the rules for reality.

Whatever is written on top of the post-it note is what the "real rules" are. Oh, you have PASTA HP now? Sure. That's how that works. You've always had PASTA HP. D&D is well known for giving fighters PASTA HP. 

The PCs can beat the Derros up (in the game world) and break their false machines to erase (or rewrite) what is written on the post-it notes. But the notes can't be removed until the plague is cured.

The plague is cured by obtaining, in real life, a copy of the Veins of the Earth. Then, rip the pages out from 167-172. The Derro no longer exist. They can no longer hurt you.

Monday, March 11, 2024

Four Rival Adventuring Parties

I love rival adventuring parties. I half use them to express ideas I have for characters in games I'm GMing. "I wish I could play this." Here are four that might appear in the first supplement to His Majesty the Worm: the dungeon named The Castle Automatic

Rival Adventuring Parties

The Debtors

A team of debt-bound dwarves. They travel under the cover of a giant's brass helmet. They speak Cant exclusively. They warn about the presence of any traps in this room (less out of kindness and more out of fear that the players will set them off while they're still nearby). 

Quest: They are searching for the Coin of Pentacles to pay off their collective debt. They will pay 20g for solid advice about how to navigate the Palace of Pentacles. 

The guild consists of:

  • Sam Miner the necropolis architect (dwarf, Path of Cups) - De facto leader. Fights with a mattock. Knows a lot about dungeon design due to his background in building trapped necropolii for other underfolk. Owns Zach, a sassy canary.
  • Albus Sapper the sapper (dwarf, Path of Cups) - Fights with a flamberge. Pyromaniac. In possession of a homemade bomb and is looking for an excuse to use it. 
  • Ethel Tinker the trapsmith (dwarf, Path of Pentacles) - Fights with a crossbow. Professional to a fault. Just there to do a job.
  • Sue Hauler the dumb muscle (dwarf, Path of Swords) - Fights with a maul. Loves her mates. Loves her beer. Loves her work (bashing heads and hauling heavy shite). Simple as. 

Mr. and Mrs. Not

Gnomish husband and wife duo who are looking for rare and exotic flowers. They travel with orcish guards who are paid to protect them. 

Quest: Exotic flowers. They are willing to pay high prices for flowers from the Castle Automatic depending on how rare they are: white lotus from XX - Ossuary (500g), roses from XX - Bower (25g), Fidelis Rose from area XX (1000g), and black lotus from XX - Pleasure Dome (50g).

The guild consists of:

  • Mr. Waste Not the cantankerous naturalist (gnome, Path of Wands) - Fights with a wand. Grumpy all the time. Traditional and conservative.
  • Mrs. Want Not the garden witch (gnome, Path of Cups) - Prefers not to fight, but will wield her cast iron pan in a pinch. Kindly. A good cook. 
  • Maer Finrydottir the mage killer (orc, Path of Pentacles) Fights with a pair of daggers. Always wears iron armor because she hates the feeling of magic being cast on her. Utterly in love with Peta. 
  • Peta Ulysdottir the pathfinder (orc, Path of Swords) Fights with a razor boomerang. Half hyper competent, half lazy. 
  • Oyster the dead one (orc, Path of Wands) Fights with a bone club. Came back from the dead once and came back weird. 

The Penitent

Templars, sponsored directly by the Cult. Believe themselves to be on a holy crusade. On one hand, they see themselves as agents of a temporal authority in the otherwise lawless space of the Underworld, which makes them come across as true dicks. On the other hand, they practice what they preach and will treat others fairly. They hate ogres, the undead, and vampires, and will always join in on battles against these opponents.

Quest: They are searching for the Chalice of Life, which they believe to be the sacred Grail of Mythrys. 

The guild consists of:

  • Philo "The Confessor" Heartsbane the inquisitor (human, Path of Swords) - Fights with a warhammer. Cautious and disciplined. Holds his guild to high standards. Comes off as joyless. 
  • Dyvin Minor "The Shield of Faith" Heartsbane the knight  (human, Path of Swords) - Fights with a mace. Lives in the shadow of his father. Overeager to prove himself.
  • But-by-Every-Word-that-Procedeth "The Holy Demon" the convert squire (troll, Path of Swords) - Fights with an iron-studded war club. Struggles with their identity as a troll living among humans. A true convert. Rich, baritone singing voice. 
  • Sister Miriam "The Water Bearer" Ormulon the war nun (human, Path of Cups) - Fights with a censer flail. Skilled healer, bad bedside manner. Has an incredibly grim view of sorcery. 

The Ones

A group of failsons who each believe they're the chosen one. 

Quest: They are searching for the Perfect Sword. Each member believes they are the true inheritors of its legacy. If they ever actually get within sight of the sword, things will get dark fast. 

The guild consists of:

  • Kinwa Too Tall the dashing bravo (high elf, Path of Swords) - Fights with an archwood sword. In love with the nymph, Imaga, who lives in a different section of the Underworld (you wouldn't know her). 
  • Franklin "The Wart" Noname the mage knight (human, Path of Wands) - Fights with a quarterstaff. Marked with a solar-burst like burn on his hand. Dismissive of dangers because he believes himself to be the child of prophecy. 
  • Bingo Beekeeper the people's burglar (halfling, Path of Pentacles) - Carries a hive of trained bees on his back. Fights by directing the bees to attack his enemies using two signal wands. Comes from a noble background, but dedicates his life to helping the poor and unfortunate.

Tuesday, February 6, 2024

Four Tips to Drive Role Playing

Dungeon games have a historical bad reputation of being all about kicking down doors and looting treasure. But even games with the most traditional trappings of bash-n-loot fantasy can have vivid, funny, interesting, memorable roleplaying. If you need a bit more in your games, try these tips.

Use Reaction Rolls

By default, nothing should be outright hostile. In The Hobbit, the three trolls Bilbo came upon felt sorry for him and wanted to let him go (at first). Use Reaction rolls to see if the dragon is hungry, angry, sleepy, amused with your antics, or otherwise. Every encounter is then a roleplaying encounter by default, unless the dice and player actions determine differently.

Meaningful language rules

Choose language rules that actually facilitate talking to lots of stuff. Go beyond letting players choose at random from "Aquan" and "Goblin." Have a tight list of 3-5 languages that everything in your setting speaks. Even when an NPC is hostile, understanding their language is a boon. The NPC can shout orders to their minions, giving you a chance to prepare and react. The NPC can speak out loud about what its motivations are, giving you a chance to parley instead. The NPC can talk about different sections of the dungeon, such as a trap in the next room it's trying to push you towards, which gives you information to work with.

Talk to everything

No seriously, you can talk to everything. Kick "languages" up a notch by making almost everything verbal. Campfires. Doors. Skeletons. Change the paradigm of what can speak and what language is for rich roleplaying experiences.

Recurring NPCs

I put recurring NPCs on my random encounter table. These entries can expand out to a secondary table, like "Rival Adventuring Party, roll again on the Rival Adventuring Party Table." Then, the players see the same merchants, the same witches, the same rivals, the same Big Bad a few times, learn their tactics, develop relationships, etc.

Monday, January 29, 2024

Metroidvanias and Megadungeons

Sometimes, I see the question: "How do I run an RPG like a metroidvania/like Hollow Knight/like Blasphemous?" 

I'm making this blog post mostly so I have something to link to when I encounter it.

Definitions of terms

What is man? A miserable little pile of interconnected, gated dungeons.

"Metroidvania is a subgenre of video games focused on guided non-linearity and utility-gated exploration. The term, popularized by video game critic Jeremy Parish, is a portmanteau of Metroid and Castlevania." - Definition from the sidebar of

"Some common characteristics and philosophies for a mythic underworld or megadungeon (keep these in mind when creating your dungeon):
1. It's big, and has many levels; in fact, it may be endless
2. It follows its own ecological and physical rules
3. It is not static; the inhabitants and even the layout may grow or change over time
4. It is not linear; there are many possible paths and interconnections
5. There are many ways to move up and down through the levels
6. Its purpose is mysterious or shrouded in legend
7. It's inimical to those exploring it
8. Deeper or farther levels are more dangerous
9. It's a (the?) central feature of the campaign" 
- James Cone, Philotomy's Musings

Locks and keys

To open the door, you must do what needs to be done.

The basic gimmick of any metroidvania is that players can see pathways that they don't yet know how to access. This is done through a series of locks and keys.

Locks are anything that prevent the players from making progress. 
  • They can be spatial—something is separated from you by distance
  • They can be physical—something can physically block your progress
  • They can be temporal—something only happens at a specific time or after time is spent on a task
  • They can be environmental—you need to overcome the limitations of the human body
  • They can be interpersonal—you must convince the NPC that you’re worth helping
The simplest example of this is a locked room. You must find the key to enter it.

But the principle applies to much more than that.

Entire sections of the dungeon can be locked. Example: You cannot journey through the Drowned Marshes without waterbreathing.

Non-player characters can be locked. Example: The ghost of the nurse will not help you until you give her the skeleton of the fair haired girl buried in the First Graveyard.

Specific items can be locked. Example: You can’t pick up the Communion Bread and Wine until you have first confessed your sins.

Certain events can be locked. Example: The ghost of the baroness only appears in the witching hour.

Achieve a metroidvania effect by using obvious locks and hidden keys.

Gated, non-linear dungeons

The map from one of the metroidvanias that lends the genre its name

As a gamemaster, creating a megadungeon can seem daunting, but it’s really just a series of small, discrete tasks.
  • Break a megadungeon down to its atomic constituent parts.
  • Populate it with interesting things.
  • Connect these levels together.
It’s less making one huge megadungeon and more creating several, smaller dungeons. Each dungeon should be interesting enough to stand on its own. Here's one recipe for stocking a dungeon. Here's another. 

That said, because you are creating several dungeons and connecting them together, not every dungeon has to have everything. Each dungeon's theme will be reflected by the choices you make while stocking it. One dungeon could be completely devoid of enemies but be full of traps (like Hollow Knight's Path of Pain). One dungeon could be free of both traps and enemies--a rare moment of quiet and place to rest in the center of the megadungeon; a camp between the over world and mythic underworld. 

Create locks between these constituent dungeons. Place their keys in places that clever PCs can find them.  

One powerful technique is to show the players they haven't unlocked yet right off the bat. Many metroidvanias do this in the first stage to establish the pattern of explore -> find a roadblock -> explore some more -> return to bypass roadblock.

This is what Sersa Victory calls a "Foreshadowing Loop"

Another powerful technique is to show a key behind a difficult puzzle that can be ignored -> the lock that blocks the PCs' progress that contains a clue for the puzzle. This lets players muse over the puzzle in context. See the puzzle, see the blocker, get a clue, backtrack to the puzzle itself.

The non-linearity of the maps and expanse of content works to your advantage. Because there are many ways to go, you can afford to get "stuck" on a puzzle. In linear dungeons, a puzzle that blocks the way forward can cause hours of frustration for player and GM alike. In non-linear dungeons, it's not a problem. The players can leave and continue to play and explore. If someone has an insight or if a key is later discovered that fits that puzzle's lock, they can return. Also, because of the nature of the medium, there should be many ways to solve any puzzle. You're not just trying to guess what the game wants you to do--you're finding logical solutions that work within the context of the game world.

"Story" and level design work together

As you play Hollow Knight, the bright Infection spreads.

In many RPGs, and especially in games of this format, the "story" is what happens at the gaming table.  To support this, GMs should imbue the megadungeon with lore seeds that bloom when cared for. 

That is to say, do not create a plot--explicit things that you think are going to happen or actions the PCs will take. Instead, create the backstory of the different dungeon levels/overall megadungeon, and let that shine through your choices of traps, monsters, treasures, and special areas. 

Embed lore and information into each level. A trap can reflect the theme of what it guards or its maker. Treasure might depict the scene of a historical or recent event. The gold might be minted with the faces of ancient goddesses. Ruins might have partial Rosetta Stones that allow the translation of unknown and forgotten languages. Let your lore shine in dark places.

Ultimately, players might ignore all this as just a bit of flavor. And that’s okay—the game is ultimately about the player’s choices. But, at the same time, if you repeatedly hit your players over the head with bits of lore, they’ll begin to form connections. Things that seemed like one-off bits of description will suddenly be remembered. Players will have “aha” moments. They’ll feel like Sherlock Holmes—and that’s fun for everybody.

Ability-based exploration 

In Blasphemous, you upgrade your thorn-covered sword, Mea Culpa, by finding new altars.

Sometimes its okay having the blue dungeon gated behind the blue door which is opened with the blue key. But a key feature of metroidvanias is to unlock progress through ability-based exploration: Samus learns how to drop bombs so that her morph ball bounces up into the tunnel, Alucard learns how to become a mist to phase through gates. It's more interesting to open new levels because the PCs gained a new ability that opened up a new mode of play for them.

There are some abilities that work better in a dungeon-crawling context and some that work less well. The same is true for metroidvania-esque exploration.

Abilities that are bad for metroidvania-megadungeons

In every old-school module, especially tournament dungeons, there was a big list of spells that just didn't work. There's a reason for that.

Passwall: Avoid any ability that removes the essential structure of locks and keys from the dungeon. Passwall (and equivalent abilities) is like that glitch where you pressed Start during screen transitions in Legend of Zelda: Link's Awakening--you bypass sections of the dungeon, but you're breaking the experience.

Jumps and spider climbs: It feels counter-intuitive. Getting a double jump works so well in video games! But TTRPGs have different modes of input than video games. When the GM says: "Ha ha, you cannot reach the door on the ledge because it is, like, 15' up," the players are well within their rights to say "Oh, uh, okay we just climb the wall" or "We have a grapple" or whatever. Putting your next level on a super slick unclimbable wall just feels cheap.

(Although, having written this, I just got an idea for an entirely upside down dungeon that's only really navigable if everybody has spider climb. Although I think you'd get a nose bleed after a few minutes. I guess my point still stands.)

Flight: I'm not an advocate for flight in dungeon crawling games. Too many great puzzles and environments only have tension because the players can't fly right over them. Putting a dungeon in the sky and then giving your player's flight is cool, but comes with a cost. It's better to unlock the sky dungeon through taming a friendly roc or climbing a giant's fishing line. 

Teleportation: Teleportation between set points is fine. But the ability to teleport small distances (i.e., through doors) will disrupt the most essential locks (doors) in a megadungeon. Intangibility has the same problem. 

Abilities that are good for metroidvania-megadungeons

Giant size / giant strength: Moving huge boulders blocking a door is a classic. It's even better if you have to move something that's inhumanly heavy: cranking a giant winch, pushing open a giant door, winning an arm wrestling contest with a giant, etc. 

Tiny size: Alice in Wonderland style, shrinking into a mouse size lets you enter through small doors or navigate doll house dungeons. 

New languages: New languages can allow you to traverse the dungeon in dramatically new ways. It might turn enemies into friends--once you can speak to the kobolds, you can negotiate passage through their territory. It might allow you to solve previously unsolvable problems ("Speak FRIEND and enter? We had this all wrong.")

Healing: Sections of the dungeon can be gated by dangers that you can eventually mitigate. For example, you can only survive the Poison Swamp once you gain the ability to Remove Poison. Or a particular type of healing might be needed to bypass a lock: a door can only be opened if the demonic sword pinning it shut has Remove Curse cast on it. 

Damage immunity: Relatedly, some dungeons can be gated by hazardous environments. Entering the Dragonfire Forge deals 1 damage by heat every turn. If the players gain Rings of Resist Fire, they can traverse this environment. Or perhaps they can enter the Poison Swamp if everybody gets a gas mask. 

Elemental damage: Even without 5E style damage types, puzzles that deal with different types of elemental effects "read" well at the table. To open the door encased in ice, you need to cast a fire spell. To charge up the elevator that takes you to a new level, you need to cast lightning bolt on it. 

Water breathing: Water is a great, natural barrier. Water breathing allows you to descend to dungeon levels at the bottom of a lake or traverse the underground river.

Further reading

Sersa Victory has written about the patterns of "locks" and "keys" in their supplement Cyclic Dungeon Generation. It's very much worth checking out.

Game Maker's Toolkit has a Patreon post analyzing Zelda dungeons, which provides a very handy dungeon graphing system.

I wrote about my procedures for making a megadungeon in a supplement called Dungeon Seeds. This no-frills, low-art draft went on to become a chapter on megadungeon creation for my upcoming game His Majesty the Worm. 

If you sign up for my mailing list, I'll send you Dungeon Seeds for free (and let you know when the full game is available for purchase!).