An encounter designed for 4-6 6th level D&D characters. It is just a nice stroll through a cave, with a few potentially deadly complications. If you are a player then stop reading here.

Summary

The party need to make their way through a set of caves that are filled with crystals. Making any sound will cause echoes to start which can cause harm to those in the caves. The party will have to try to move through the caves silently.

When I ran this it was a travel encounter with the party needing to get through the caves to reach their destination without being noticed by guards. It should work as well as an exploration encounter if the party are sent to recover an object from one of the chests hidden in the caves.

Region

Barrier Mountains

This is set in the eastern Barrier Mountains and is focused on the party making their way through a dangerous cave complex to reach the Magrannor Halls.

Green Vale

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Map of the Green Vale showing the main settlements.

Crystal caves

The encounter takes place in Thilden caves, which are below the dwarven settlement of Khezdur. The caves are generally regarded is impassible, but for those brave or foolish enough to make their way through the caves they do allow access to Khezdur without going through the mines or Varbeck’s Walls.
While there is a map included for this encounter it should work with other layouts, or even without a map.

Preparation

This could be a fairly vicious encounter if the party do not prepare for it so they should be given the opportunity. Thilden caves are well known in Khezdur and any dwarf from the region will know about them which should make gathering information easy. The key pieces for the group to know are:

• The caves are filled with crystals which emit a faint glow.
• Making noise in the caves produces echoes which can cause death or insanity.
• Attempting to break any of the crystals in the mines can have dire consequences.
• Even touching the crystals can be enough to set off the echoes.

Obvious preparations include:

• Removing noisy armour (of the kind that imposes disadvantage on stealth).
• Securing any gear on the outside of their packs that might jangle.
• Gagging any bards and taking away their instruments.
• Using cloth or wax to block up their ears - resistance to sonic damage but they will need another way to communicate like Combat Sign.

Arriving at the Caves

The journey from the surface to the entrance of Thilden Caves takes about 2 days. This provides plenty of time for a party to discuss their plan (which they will clearly not leave to the last minute when they arrive at the caves). As they get near the entrance the party will see a faint glow coming from the tunnel ahead.

You see a faint glow coming from beyond the next bend in the tunnel you are following. The light is steady but the colour seems to shift between red, green, yellow, and purple. Peering around the corner you can see that the tunnel widens into a cavern that is filled with crystals of different sizes and colours. In the faint light emitted by the crystals you can see that there is a narrow path that is mostly free of crystals winding it's way deeper into the cave.

The crystals emit a dim light that will allow creatures with darkvision to see normally throughout the caves.

Moving through the caves.

Making a sound

In order to move through the caves the party will need to move slowly and carefully. This should be done in combat order even though there is no combat.

• A character moving at 1/2 speed needs to make a DC 12 - Dexterity (stealth) check using their action each round to avoid making a sound.
• Moving at normal speed increases the DC to 16.
• Dashing increases the DC of the stealth check to 22. The stealth check requires an action which may limit which characters can safely move in this way. A DC 15 Dexterity (acrobatics) check is also required every time the character changes direction to avoid brushing against the crystals.

The stealth checks are to avoid making noise and to avoid brushing against the crystals. Abilities/items that grant bonuses/advantage to avoid being seen do not apply to these checks.
Making a sound in the caves will cause echoes to start in a section of the caves. Different levels of sound will increase the echoes by a number of levels as shown in the following table.

*Casting a thunder/sonic based spell will trigger a tremor and psychic backlash which will kill everyone in the area. No saves, no damage rolls, just death.
Casting silence in an area will prevent sound based echoes from starting (though the casting itself could trigger some), but brushing against a crystal or breaking one will still escalate the level of echoes. Silence will not suppress the psychic or ability damage.

The Echoes

The echoes will deal damage to anyone in that section. The amount and type of damage will depend on the level of echoes that have been triggered. The echoes in a section last for 1 minute per level of echo reached.

Ability score damage from the caves is recovered at 1 point for each ability for every 24 hours that pass. In Tremanac we use the rule that you need an ability score of at least 10+spell level to be able to cast a spell.

Once the echoes in a cavern reach at least level 4 they will increase the level of echoes in adjacent chambers by 1. This can create a feedback loop with adjacent sections. The increases in adjacent sections only happens when a section reaches a new echo level of 4 or greater.

Once the psychic damage starts those affected will begin to hear faint whispers in their minds. These start as being just on the edges of the character’s awareness. They will be aware that there is a whisper but not be able to hear it. As the intensity of the echoes goes up, the whisper will become clearer. The whispering voice will offer knowledge and power if only the character is willing to stay and listen. The voice will promise secrets, magic, and lost lore, becoming more specific to a character as the level of echoes increases. The voices may not be lying about granting secrets and power but anyone remaining in an affected area is unlikely to live long enough to make use of them.

Cave map

Crystal Caves by Tanai Cuinsear

There are two pools in the caves which are connected via a submerged tunnel. A character could make their way between the caves through this tunnel but it requires a DC 20 Dexterity (stealth) check to enter and exit the water without creating a sound.
There are several chests resting between, or buried in, groups of crystals. These chests were placed here by those who listened and obeyed the whispers produced by the caves. Each chest is locked, requiring a DC 12 Dexterity (thieves tools) check to open. The chests are also trapped, requiring a DC 13 Intelligence (Investigate) check to find and a DC 13 Dexterity (thieves tools) check to disarm. The traps do no damage, they just create a loud sound that increases the echo level by 3.

Ending the Adventure

The encounter ends when all the party members manage to make it into the exit tunnel. The effects from the crystals will end 10ft into the exit tunnel. It is then just a half day journey to Khezdur.

Related Locations

• Thilden caves

Related Session Reports

• Echoes in the caves

Colours of text

Notes from when I ran the adventure

Read this aloud

Self-Inflicted

With no monsters involved the only dice rolls required from the DM are for damage. Everything that happens to the player characters is a result of their actions and you can reinforce this by having the player whose character caused an echo roll the damage for that section.

Alternative Awards

If characters experience the onslaught of level 5 or higher of the echoes and survive the caves then they may develop psychic powers. This could be learning the cantrip Mind Sliver, or getting a once per long rest use of Dissonant Whispers. One of the player characters in my game developed his powers as an Aberrant Mind following a childhood misadventure in the caves

Sonic Damage

The term sonic damage is used throughout this encounter to represent that it is from the echoes. It is the same as thunder damage and Thunder resistance will reduce this damage.

A watching Mind

Once the psychic damage starts then spells that detect the presence of thoughts will pick up an alien mind behind the attack. This mind is too different for any coherent thoughts to be read but it is definitely there and watching.

Telepathy and the caves

If any of the characters have a telepathic link set up with someone else, then that person will take half the psychic damage received by the person in the caves.

The region map was created in Wonderdraft. The cover image and map of the caves were created in Talespire.