What This System Tries To Do

• The Underdark is vast and dangerous. To try and map it, is folly. Navigation must be done differently; by using all of your senses. Tunnels twist and turn, and may not lead into the direction you were hoping for.
• This system tries to capture the vibe of the Fellowship of the Ring traveling through the Mines of Moria, more specifically, Gandalf wondering which path to take.

“Oh! It’s that way.”

“He’s remembered!”

“No, but the air doesn’t smell so foul down here.”

Gandalf & Merry

Just Follow Your Nose

A core pillar of this system is that the party has some idea of where they have to go, based on their senses. This can work several ways:

• One of the party members traveled here, a long time ago. They don’t remember individual tunnels, but they do remember what they sensed walking here – cold air followed by the smell of sulfur, etc.
• The party has received a cryptic description in an old Dwarven tome, or perhaps Dwarves don’t map out the Underdark at all, and give out sensory directions by default – “climb upwards until you are greeted by the flow of warm air, then turn towards it” is a more practical description than numbering the number of crossroads.
• The party has a specific goal inside the Underdark, that has a strong sensory ‘milestone’: The smell of fungus spreading from an underground forest, warm air coming from a subterranean hot spring etc.

5 Senses, Underground

Sensory input in caverns can take the following forms:

• Sound. The distant crashing of an underground waterfall, the wind howling through tunnels, or complete lack of sound.
• Smell. The smell of rot, fungus, sulfur, salty water.
• Sight. Particular mould, discolorations of tunnels, bioluminescence.
• Touch. A flow of air from or to a tunnel, air being warm or cold.
• Taste. Ew. I guess you could lick rocks to taste them?

Sensory Instructions & How To Follow Them

There are a few ways to play out sensory instructions and how the players interact with them.

Abstract

Instead of making actual instructions, the players are simply told that they have a set of instructions.

Whenever players arrive at an intersection or crossroads, they can make a relevant check (Perception/Investigation/Survival/Nature etc.) to deduce the right tunnel based on their (abstract) instructions. A DC 10 would represent a forked path, a DC 20 would represent many paths to choose from.

On a failure, the party picks out a path they are sure is the correct one, but isn’t. They wander that path for a while before realizing their mistake and heading back, which takes 1d4 hours.

On a success, they successfully deduce the correct path (“You feel fresh air coming from the left tunnel, which corresponds with the instructions.”). The DM marks a success, with their objective being reached in a certain number of successes.

Semi-Abstract

Here, the players do receive actual instructions; these can look like this, for example:

Follow the sound of crashing water until you reach the underground waterfall. From there, follow the smell of fungus, until you feel a cold wind blow. Walk against this flow of cold air to emerge at your destination.

These instructions would play out like this:

1. At the first 3 crossroads, listen for the sound of water. This is made at DC 20, 15 and 10, as you get closer to the source. Succeeding the check means you perceive the sound and pick the right tunnel, failing the check makes you pick a wrong tunnel and wander around before heading back, taking 1d4 hours.
2. The cave containing the underground lake and waterfall has 4 tunnels leading elsewhere. A single DC 20 Wisdom (Perception) check captures your best efforts to choose the correct tunnel, a success leading you onwards, a failure costing you 1d4 hours.
3. For the next 4 crossroads, the players are to be on the lookout for the sensation of cold air. Each of these crossroads feature only a single path splitting off from their current one, so we set the DC relatively low, to 10. Failing means 1d4 hours lost.
4. Finally, for the home stretch, 2 intersections at which to follow the flow of cool air, at DC 15, with multiple paths to choose from.

Concrete

This is the most elaborate method. It differs from the semi-abstract version in two ways:

• The DM has an absolute map of the Underdark. The players do not, but can make one themselves, if they wish. Choosing the wrong path does not result in “1d4 hours lost” – it means that the players are not where they think they are, giving the possibility of them getting horrendously lost.
• The instructions are a puzzle in and off themselves. You can make this as complicated as you want (or just use the Semi-Abstract instructions). This means the instructions look something like this:
  • Always descend, unless the air reeks of sulfur
  • When presented with two paths of equal size, choose the left one, unless from it a cold wind blows
  • After going straight for 3 intersections, a left or right must be taken
  • etc.
• This is effectively turning navigation into Einstein’s Riddle, making it so that even when the party has all sensory information about the available tunnels, the right choice requires thought.

Back The Way You Came

Backtracking or revisiting previous tracks should be easier than plotting new routes. Implement this by replacing all the sensory deduction with an Intelligence or Wisdom check with Advantage to emulate the memory a character has of the path.