Wednesday, April 12, 2023

THE UNDERCLOCK: Fixing the Random Encounter

For a long time, I had the same random encounter table rules as everyone else:

Every 3 rooms (or 30 minutes of exploration time), roll a d6.  If a 1 comes up, a random encounter happens.

After a couple of years, I even added

If a 2 comes up, the players get an Omen--a clue about an upcoming encounter.

In practice, this . . . never quite felt the way that I wanted it to.  And it's taken me a decade, but I think I know why.

by Peter Mullen

First, look at the math.

This works out to 1 random encounter every 18 rooms, and one omen every 18 rooms.

We probably got through 4-10 rooms per session, so random encounters don't pop up very often.  (I don't use a lot of empty spaces.  Most of my rooms have something to interact with.)

Some people roll a random encounter every room (including every long hallway).  This feels a lot better, but it also means that back-to-back encounters are not infrequent, and it feels too unpredictable.

Second, look at how omens are supposed to work.  You're supposed to find the omen, which (a) builds suspense and (b) lets the players prepare themselves, prior to the encounter itself.  Omens weren't happening with enough frequency.

Third, consider the stated goals of random encounters.  (Time pressure, versimilitude.)  Random encounters are not the best way to achieve either of those.  Often, when random encounters occurred, they felt disruptive.

Anyway, I am giving up on:

  • tracking torch durations
  • random encounter checks

I think I have something better.

Design Goals

Time Pressure

The four most vital currencies in a dungeon crawler are:

  • Time
  • Information
  • Hit Points
  • Expendables (potions and spells)

It doesn't feel like a dungeoncrawler to me unless all four of those things matter.

The reason why we track torch depletion and random encounters is because we want a source of time pressure.

I like games where time a critical resource.  Players can't read every book and tap every brick.  They need to be tactical in where they spend their attention.  

This is good!  It raises the skill level for dungeoncrawling and makes players scrutinize my dungeon for clues.  Do they have reason to think there's a hidden door in this room?  If so, they'll spend some time searching.  If not, I'll keep moving.

But I don't think random encounters are very good at this task.  Random encounters don't put much time pressure on players unless they're very frequent, and at higher frequencies (1/room or greater) they feel too uncontrolled to me.

Suspense vs Surprise 

Surprise is when the alien suddenly attacks.

Suspense is when you know the aliens are getting closer.  You are running out of time, and you are running out of ammo.  I think that most DMs want their players to feel suspense, more than surprise.

A truly random roll (a flat 1-in-6 chance) doesn't offer much suspense.  Surprise, but not suspense.

To increase suspense, you want the players to feel like they're running out of time.


I'd also argue that random encounters don't do much to make a dungeon feel like a living, breathing place.  Random encounters frequently feel out-of-place (especially when they're not well-localized to the story and the rooms around them).

Time Pressure

Random encounters are meant to be a tax on character time--don't dawdle in the dungeon or you'll lose HP--but they often feel like taxes on player time--don't dawdle in the dungeon or you'll be stuck in some pointless combat for 45 minutes, when everyone would have more fun with 45 minutes of exploring new rooms.

If the only goal was to put time pressure on the players, you could replace all random encounters with a cold wind that deals 1d6 damage to everyone (save for half) then moves on.  The players are still penalized for taking too long, and then everyone can get back to dungeoneering.

(These last two points are really just complaints about how the random encounters are written, rather than the mechanic that produces the events.  I'm including them only out of a sense of completion.)

by Peter Mullen

The Underworld

The Underworld is not just a basement or a cave.  The Underworld is a place that hates you.  It is hostile architecture.  It hates you in a way that only the blind tonnage of stone and cold air can have.  It hates your lively blood.  It hates the sunshine warmth still lingering on your skin.

Live there long enough, and the Underworld can learn to tolerate you.  You will grow pale and cold and strange, like the other inhabitants of that place.  The long years will render you smooth and inoffensive, like a pearl held in the mouth.  The Underworld's irritation fades and scabs over.

But this doesn’t apply to delvers.  They dig greedily and they dig fast.  They are hated the most, and this hatred is felt as soon as the Underworld is entered.

Explorers have a myriad of names for this feeling of supernatural dread.  The Claws of the Underworld.  The Black Rat Whisper.  The Fosydra.  But only one name suits our purposes here:

The Underclock

It starts at 20 when you walk into a dungeon.  When it reaches 0, an Encounter happens.

You will periodically roll a six-sided Underworld die and subtract it from the Underclock whenever the party expends time or noise.  Examples of actions that provoke an Underclock Roll:

  • Exploring a new room (including long hallways).
  • Moving through 3 already-explored rooms.
  • Lingering or searches.
  • Making noise (e.g. kicking down a door).

NOTE: the noise of combat doesn't normally contribute to the Underclock.

Some more facts about the Underclock:

  • The Underworld Die explodes.  If you get the maximum value (e.g. a 6 on a d6), immediately roll it again and subtract the result from the Underclock.
  • If the Underclock drops below 0, an Encounter is triggered.  After the Encounter resolves, the Underclock resets to 20.
  • If the Underworld Clock equals 0 exactly, it resets to 3.  The Underworld's attention is elsewhere, momentarily.
  • If the Underworld Clock equals 3 exactly, a Shadowing Event occurs.  So the clock reaches 0, it bounces up to 3 and a Shadowing Event occurs.  (These are just omens/spoor/clues, more or less.)


You can rest at any time.  You'll get all of your HP back after you do.  There are 3 prices.

1. The first cost is time.  You’ll have to roll some Underclock Rolls.

  • If you are resting in the middle of a well-traveled location, make 3 rolls.
  • If you are in a secret room that no one else knows about, you don’t need to make any rolls.
  • Everything in between is either 1 or 2 rolls (defaulting to 2 rolls if you aren’t sure).

These rolls explode normally.  If you are interrupted by an encounter, you’ll need to start over.

2. The second cost is a ration.  Cross it off your inventory.

3. The third cost is attention.   Each time you rest in the dungeon, increase the size of your Underclock Die.  d6 -> d8 -> d10 -> d12 -> d20.  Each time you spend a night sleeping on the surface, decrease the Underworld die by one size, down to a minimum of d6.

Everyone knows that you shouldn’t eat anything in the Underworld. 


The Underworld hoards its treasures.  It hates the idea of its gold returning to the surface.  Far better that the treasure remain in the possession of one of its inhabitants.  Someone who will carry the gold until it dies in another dark corner.  

Whenever you leave the dungeon, the dungeon degenerates.  Things may change, the dungeon may restock, but most critically, the treasure depletes.

Every time the players exit to the outside, the biggest treasure pile in the dungeon loses 20% of its value, stolen away by agents of the Underworld.  (Sometimes this is an elder dungeon spirit, sometimes this is just a goblin with a boot full of loose coinage.)

Integration with Other Systems

The Underclock replaces torch depletion and other random encounters. 

The party still needs 1 lit torch for every 3 adventurers.  Everyone gets -1 Initiative for every person not sufficiently illuminated.

The maximum Underclock value is reduced by 1 for every point of Encumbrance held by any member of the party.

The Underclock replaces regular durations of spells, etc.  30 minutes = 3 exploration turns = 10 points on the Underclock.


It takes an average of 5.9 Underclock Rolls to generate an encounter.  There is a 0.3% chance of the Underworld clock generating an encounter in a single roll (really a series of exploding d6s).

This is roughly comparable to the old "1-in-6 encounter chance every 10 mins".

Encounters are Shadowed approximately 33% of the time.  The other 67% of the time, they are not preceded by any Shadowing.

I do love that it is countdown, which makes the time pressure feel much more palpable at the table.  I would say "you've been in this dungeon for 3 hours now" and no one would care.  I would roll a random encounter check and players would glance over.  But people pay more attention to the Underclock.

"But Arnold, doesn't this allow players to game the system?  If the Underclock gets down below 6, won't they just hunker down somewhere safe until it goes below 0?" - You, probably.

Yes, of course the players will be more cautious when the clock is low, and slightly bolder when the clock is fresh.  That's kind of the point.  There's a texture to time that didn't exist before.  The players are supposed to be fully aware of it because it's a tangible-and-fluctuating measure of risk.  And the characters are supposed to be fully aware of it because they all develop heart murmurs when they draw the Underworld's attention.

If the players hunker down somewhere safe to let the clock expire, consider it an organic replacement for the Exhaustion mechanic that you see on overloaded encounter die mechanics.  (I like overloaded encounter dice.  I like the Underclock more.)

The integration with rests and HP is also something I like.  You can get all of your HP back easily, but at the cost of making the dungeon more dangerous.  There's an interesting decision here.  Should the players press, their luck?  Or break for lunch?

Similarly, there's another tension when deciding when to return to the surface.  The thought of a goblin walking off with 20% of their loot is very motivating.  The decision should be interesting and impactful (like most of the core decisions in an OSR game).  (Of course, if your game sessions are bounded by leaving the dungeon, this extra motivation is not needed.)

I'm also trying to simplify torches and encumbrance to the point where they are still important, but I want to move them out of the spotlight.  Players shouldn't spend much time or thought on them.  (I don't think I've ever had an interesting, impactful gameplay moment arise from torch depletion.)

Simple Mode

The Underclock counts down from 20 to 0, losing 1d6 every exploration turn.  An encounter happens at 0.

If you want to add some version of "the dungeon gets harder when you rest" or "treasure vanishes when you leave for the day" feel free implement your own version, or not.

Math Discussion

I wrote some Python to roll dice for me.  (Feedback welcome.  I've never shared any code before.)

Here are the numeric results.  

Here are the visualizations:

These are almost exactly the results I wanted when I started looking at candidate mechanics.