Depth crawls
Exploring The Stygian Library, The Vast, and an Interstellar Mall using 1d20 + Depth
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Last week we looked at Ten Lessons from Five Games, with Star Wars: Shatterpoint and John Company: Second Edition topping out the poll for the most inspiring game.
This week we are exploring the depths with an interesting TTRPG mechanism: Depth Crawls!
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The Stygian Library
My first exposure to a depth crawl was The Stygian Library written by Emmy “Cavegirl” Allen, published in 2020. The introduction notes that this is mechanically similar to Emmy’s earlier Gardens of Ynn:
Well, people seemed to like Gardens of Ynn. So, here’s more in a similar vein. Ynn was outdoors, this is indoors. Different locations and monsters, but the same basic tone and structure.
Both Gardens of Ynn and The Stygian Library are examples of depth crawls. It’s a way of procedurally (or randomly) generating a point crawl style adventure for use with a TTRPG.
As players descend into the dungeon or other setting, they increase a value called Depth. The player’s Depth starts at 0 (the highest level of the dungeon) and increases by one each time they go deeper into a new, lower level. At ten levels down into the dungeon, their Depth would be 10.
Using The Stygian Library as an example, here’s how it works:
Location: Players roll 1d20 + Depth on a Location table to see where the next door leads. This might be an Entrance Foyer or a Skeleton Collection as examples. Each location has it’s own description and possibly another table of random details.
Details: Players then roll 1d20 + Depth on a Details table which adds something of interest to the location, such as a fireplace or turning gears. Like the locations, each detail has it’s own description and possibly more random tables to use.
Random Event: Next they roll 1d20 or 1d12 on the Random Events table. This result might trigger additional rolls on Encounter or Treasure tables. It might also increase or decrease their progress through the library.
Players then decide to stay where they are, go deeper, or go back. Staying in the same place causes more random events. Going deeper increases the Depth. Going back returns the players a previous location, possibly at a lower Depth.
Going deeper
The interesting part is how the Depth increases each time the players go deeper. That Depth value is always added to the Location and Detail rolls when looking up the result on the tables. This is why the tables have more than twenty items, even though you are rolling a twenty-sided die.
The Locations table has 35 results available, but the ones past 20 are only accessible as you go deeper. Your first roll at Depth 0 would be a result between 1 and 20. At Depth 2, the result would be between 2 and 21. At Depth 5 the result would be between 6 and 25, and so on.
This allows for a random and yet structured progression system to help guide the world building. You’ll only ever find an Entrance Foyer (1) at a Depth of zero. To find Jarred Brains (31), however, you would need to be at a Depth of at least 11. The dice roll is a shifting window of 20 available items that get stranger as you go deeper.
The random encounters are structured in the same way, using 1d20 + Depth. When first entering the library at Depth 0, you might encounter a giant bookworm or a lantern-bearer. It’s only after descending multiple levels that you might find Grey Order Librarians or an Eye Sentinel.
If a value of 35+ is ever rolled, the players re-roll d20 + d10 + d6 - 2. That gives a slightly flattened distribution ranging from 1 to 34:
Making progress
The other part of The Stygian Library that is really interesting is the addition of a Progress track. Rather than just relying on Depth alone to determine what can be found while exploring, a separate Progress number is used.
Progress increases if the party advances the overall story (e.g. talking to someone important, finding a critical item), but it might also decrease (e.g. the party gets lost). There are a few results on the Random Events table that increase or decrease the Progress by a random amount.
This Progress value is used for tasks like looking for a specific book in the library. Two conditions must be met using a difficulty-based target value:
Depth must be equal to or greater than the target value minus 20.
Progress must be equal to or higher than the target value.
An item with a difficulty target value of 20 could be found at Depth 0 and Progress of 20, while a target value of 30 would require Depth 10 and Progress 30.
This mechanism adds more narrative cohesion to the world, ensuring that end-game items aren’t found too early. Instead of purely random loot, items can follow the same satisfying progression as locations.
Delving the pillars
The Vast in the Dark: Expanded by Charlie Ferguson-Avery is another example of using depth crawls:
The Vast in the Dark is an exploration setting for the world’s most popular role-playing game and takes place in a crumbling alien wasteland filled with brutalist mega-structure ruins. It includes tools for generating massive areas to explore as well as unique player options and gameplay changes to keep the focus on player choice and ingenuity over dice-rolling and chance.
While exploring The Vast and The Wastes, players might find The Pillars, an area of endless tunnels leading downward.
The loot found in the Pillars is based on a roll of 1d6 + Depth on a random table. At Depth 0 you might only find some forgotten corpses or raw lodestones, while at Depth 12+ you have a chance to find artifacts.
Events in the Pillars work a little like the Progress value in The Stygian Library, but it is incremented every time a roll is made on the Pillar Events table. That means it continually increases, with higher values being more dangerous for the players.
The Ruins also work as a depth crawl. Each time the players enter a new room, they roll two d6 (i.e. d66) for the room type. The room’s Feature (1d20 + Depth) and Encounter (1d10 + Depth) are both based on how far down they have travelled.
Treasures found in The Ruins are grouped into Something Useful (1-10), Something Special (11-19), and Something Great and Terrible (20+). Rolling 1d20 + Depth determines the group, and then each group has 10-12 items within it. While it’s possible to find Something Great and Terrible at the entrance level, it is unlikely. Only going deeper will increase the chances.
Interstellar Mega Mart
Depth crawls aren’t just for spooky libraries and dungeons. The Hull Breach supplement for the Mothership RPG includes the Interstellar Mega Mart (ISMM). Players begin at Level 1 and can Go Deeper (+1 Depth), Stay Here, or Turn Back (-1 Depth).
Locations are 1d10+Depth, and then Details and Events are a simple 1d10 roll. They continue down until reaching the Exit (19) which then leads to the first level of a new ISMM.
Progressive risk
Depth crawls are a way to add progressive risk to tabletop games. The deeper you go, the higher the chance of rolling something dangerous. For more on progressive risk mechanisms and ways to structure a game to support the narrative, check out Muffins and the risk of being eaten and Stacking the deck.
Conclusion
Some things to think about:
Depth crawls aren’t just for dungeons: The Interstellar Mega Mart shows how depth crawls can be used in any genre. The direction doesn’t need to be down, as you could use the same mechanisms for climbing a mountain (1d20 + Height) or things getting hotter (1d20 + Temperature).
Mechanisms to support the theme: It’s a tough balance sometimes to have both randomness and yet also have a coherent story. If you are generating a map, you don’t want a swamp next to a desert. Depth crawls are a way to ensure there is some sort of sensible progression to what is found by the players.
Multiple tracks or values: The way that The Stygian Library uses a Progress value to handle finding books is really interesting when combined with the basic depth crawl mechanism. Similarly the tracking of “number of rolls” in The Vast in the Dark achieves a similar purpose.
What do you think? Have you used a depth crawl mechanisms in your games?
— E.P. 💀
P.S. I’m doing a talk on Accelerating Tabletop Game Development with Python Simulations at CPOSC on Saturday, April 6, 2024. Tickets are available now.
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I love depth crawls. I used this mechanism for a horror journaling game, and it worked great to allow me to make the prompts for early in the story be relatively mundane and then get progressively more unnatural as the story unfolds. I used a “Stress Level” to track the “depth” on the prompt table that the player rolls for events.
Iko has a few nice ones for Skyrealms: The return of the She Goddess, an ecological heat-crawl and Stranded in the Skull, a depth crawl. Both of them fit one single postcard.
https://the-lost-bay.itch.io/skyrealms
Inspired by him I also made one for Pine Shallows. Grasp of the Understory, is a depth crawl into a weird hidden pocket realm, to rescue a missing teacher of the player characters.