There’s no wrong way to play a TTRPG. But is there a wrong way to write a depthcrawl?
I recently got to chat with Waco Matrixo, writer of Decagon, about depthcrawling. If you haven’t read Decagon, it’s one of Mothership’s finest (check out my microreview). Waco mentioned a plan to run Raider Bounty of Gilgamesh VII this weekend; we’re eying collaborating on a depthcrawl for Triptech3, so he suggested I check it out.
We’ll have to get it to the table and give it a play before we review it here, but Raider Bounty is a cool little two-spread module. The NPCs have some good secrets, and there are some interesting ideas, like rolling encounters at advantage or disadvantage depending on depth. The font size hurts my old-ass eyes, though. Still, it’s a lot of content in a small space, so I’m sure that was unavoidable.
Leaving aside the merits of the individual module, one thing stood out: the use of a 2d5+Depth table for the depthcrawl.
2d5 or not 2d5
Emmy Allen originated the depthcrawl with her excellent Gardens of Ynn and Stygian Library (though arguments will persist). Both operate on a 1d20+Depth system; both offer the (now fundamental) Locations, Details, and Encounters tables, with 35 entries per table.
When we kickstarted a Privatized Little Hell, we hit a stretch goal to add a depthcrawl. We went with a d6+Depth, out to 13 entries.
When Dave Kenny of Magnum Galaxy Games ran Nightmare on Somerset for us, I believe (if my memory serves) we rolled a d10+Depth for Locations and a straight d20 for Details and Encounters, with 20 entries per table.
Raider Bounty of Gilgamesh VII does 2d5+Depth, with 15 entries.
2d5? Oh, how I scoffed! Surely the module would run better at d5+Depth, or even d10+Depth. Did the writer (Chris Air of 5 Million Worlds, with a company of others) think they knew better than the established wisdom? I ran straight to the Math-Cave, to prove this upstart wrong.
Blasted Bell Curves!
The issue is simple. A single die has a flat probability ‘curve’; you’re as likely to roll a 1 as you are a 5. Adding a second die changed that curve. Consider a classic 2d6 roll; there’s a 1/6 chance of rolling a 7, but only a 1/36 chance of either a 2 (Snake Eyes!) or a 12.
When it comes to Random Encounter tables, it can make things boring fast. Suppose you have an 11-entry 2d6 table of Random Encounters, with a nice, diverse spread of things your players can find. Three numbers (6, 7, and 8) make up 16/36 possible rolls (4/9ths for you factor-heads). That means a quarter of your table makes up almost half your results. In other words, if you only hit two Wilderness Encounters in a session, they’ll both be from those three encounters about a fifth of the time.
So what happens if you apply this to a depthcrawl? Do you get the same boring predictability?
Consider Raider Bounty of Gilgamesh VII’s Events table. Events 5 and below are relatively benign. There’s a chance you might get disadvantage on your rolls from the cold, but that’s it. Things escalate quickly after 6: encounters, toxic floods, cave-ins. But how likely are you to see the sub-6 section of this table in play?
Well, starting at Depth 1 and adding 2d5, there is a 24% chance of hitting one of these zones on your first roll, 12% on your second, and 4% on your third. You can invert those percentages and multiply them to get the chance of missing all three: 64%! Remember, because the table starts at entry 3, these three entries make up almost a quarter of the whole. These are not so much usable parts of a depthcrawl as they are flavour text.
Repeat Headaches
Then there’s the opposite problem: areas you’ll hit a lot.
Obviously, with a good depthcrawl, you’re relying on the other tables to make repeat rolls interesting, so this isn’t necessarily a deal breaker. Still, consider the following.
- You must reach at least Depth 5 to have a chance of hitting the final encounters.
- You’ll be rolling this table at least five times (though Raider’s shortcut system could cut that down).
- Your most likely roll (at 20%) is a 6.
Consider Event 8, a toxic flood that damages you and sweeps you away. On your first roll, there’s a 16% chance you hit this. At Depth 2, that goes to 20%, then falls to 16% again, then 12%, and so on. If you’re one of the 1-in-6 tables that hit it straight out of the gate, there’s a 1-in-5 chance you’ll see it again immediately. Your chances of seeing it again between Depths 2 to 4 is 41%.
This is only entry 8. Entries 9, 10, and 11 are even more likely to be seen and recur.
Consider Event 11, a collapse. At Depth 5, you have a 20% chance of hitting it. In your previous four rolls, there’s a 35% chance you’ve already seen it. In your next four rolls (which it’s unlikely you’ll get to, but would take the peak of the bell curve over the 15+ climax entry), there’s a 35% you’ll see it again.
Is Flat Good?
So far, things are panning out in line with my earlier scoffing. Surely a quick check of the traditional depthcrawl math will confirm my biases. Let’s compare this 2d5-ery to a system where you start at Depth 1 and add a single d10.
There’s a 10% chance you hit that 11th entry on your first roll, followed by an identical chance on every following roll.
Your chance of hitting it before Depth 5: 59%
Your chance of hitting it at Depth 5: 20%
Chances of hitting it between Depth 6 and 10: 67%
Huh?
Lightbulb Moment
Because you’re constantly adding to the Depth, the bell curve peak goes from being a force of consistency to a gentle wave of guidance, pushing you through the depthcrawl at a steady pace. There’s still a strong element of random, procedural generation, but you’re less likely to get repeats because the centre of gravity is always moving forward. Additionally, like-numbered entries across different tables are more likely to group together, allowing a consistent tonal shift as you move through a space.
It’s true that the outlying results are less likely to see play, but that should be easily fixable by lumping a few results together. For example, entries 3-4 could contain a result, and 5-6 could be grouped as well. This might help these early entries feel more relevant, especially relative to a flat d20 roll.
What can I say? I think I’m a convert. 2d-whatever is the one true way. I was blind, and I lived in a poverty of mind and dice. Now I am a follower of a better way.
Or did I do the math wrong? Do I know anything about anything? Is there only one correct way to crawl depths? Let me know in the comments below; let’s have one of those antiquated human-to-human conversations.
Like this article? There’s been a bunch of ink spilled over depthcrawls. Check out this great Emmy Allen interview for more depthcrawl stuff. Or stay on our blog for this 5e hexcrawl advice. We’ve also got a great little MOTHERSHIP pamphlet toolkit from the last TripTech Jam; check out Millennium Trail for all your generation ship needs.