The Underdark

Underdark Information:

Here you can find all the Underdark Information that you already have and that which you have gathered so far. More Underdark Information will be added as our Heroes encounter them



Below is some simplified cave terminology to provide a basis for description on Underdark Information. These terms apply equally well to “caves” of any material.

Cavern: A large chamber in a cave.

Dead Cave or Dry Cave: A cave without running streams, lakes, or drips of water.

Gallery: A large, mostly horizontal chamber.

Lake: A standing body of water of any size. What upperworlders might call a puddle may be a lake in the Underdark.

Live Cave or Wet Cave: A cave with flowing water or a lake.

Maze Cave: Several passages and chambers interconnected, often on two or more levels.

Ossuary: A cave filled with bones. If the bones are fossils, the cave is often called a fossil chamber.

Spring: A natural upwelling of water from rock or soil. Few springs offer drinkable water; the majority of them are laden with harmful substances.

Location - Underdark Cave
Location – Underdark Cave


Underdark rivers tend to be swift, violent, and tortuous in their windings. It is a rare river indeed that flows level and smooth for more than a few miles at a time before disappearing into a deep gorge or sinkhole in a fuming waterfall.

Rivers are the great builders of the Underdark, the natural force that sculpts great caverns and brings lifegiving energy and food to sustain the Underdark ecology. Most rivers are surrounded by a halo of living caves, which can be valuable real estate indeed.

Location - Underdark River
Location – Underdark River


Perhaps the most wondrous of the Underdark’s features are the vast, nighted seas of the deep earth. Seas are saltwater bodies, not fresh, and most of them are found in the Lowerdark, though Underdark seas also occur at higher spots beneath Faerûn’s surface oceans.

While air-filled cave systems may extend for dozens or even hundreds of miles beneath the oceans above, or form air-locked siphons of extraordinary size, these features are exceptional. Most caves beneath large bodies of saltwater are simply subterranean extensions of surface oceans.

Seas tend to form in the largest of vaults, occupying caverns large enough to be miniature worlds in their own right. Like the lakes, seas offer some of the best roads in the Underdark, and many are heavily traveled.

Location - Underdark Sea
Location – Underdark Sea


A tunnel is simply a passage that connects one place with another. Most are cut by creatures, though some are the results of natural movements of the earth and other forces. Underdark races often cut very ambitious tunnels to link multiple cave systems. Though such dreary passageways may be dozens of miles in length, most are only 10 or 20 feet across. Other tunnels are the work of burrowing monsters such as Delvers, Purple Worms, and Umber Hulks. These “Natural” tunnels may be twisting, turning mazes of intersecting passages.

Tunnels are some of the Underdark’s most useful roads, but they severely restrict a traveler’s options. If you don’t like where a tunnel leads, you really have no choice but to go back the way you came. Tunnels also offer few hiding places for those who cannot blend in with stone, so often the only way to get away from a predator is to run – and hope you’re faster.

Location - Underdark Tunnel
Location – Underdark Tunnel


Sometimes natural processes form deep pits or wells in the earth. The shaft of such a structure may be carved out by water flowing straight downward in a subterranean waterfall or created by volcanic activity. Unlike a gorge or a rift, a shaft tends to be a relatively small feature6′ – 7′ feet (usually less than a bow in diameter), but it may plummet for miles straight down.

Because of their relatively small cross-sections, shafts often serve to channel air movement between disparate portions of the Underdark. In places where the conditions are extreme (for example, a shaft near a superheated magma chamber), the air movement can also be extreme. Screaming winds might roar up or down a shaft in a scouring blast that would put a hurricane to shame. Sometimes, cave systems “breathe” in conjunction with changes in the surface world above, resulting in tremendous rushes of wind in and out through shafts every day.


The higher reaches of the Underdark consist of immense networks of relatively small caves, but as a traveler descends, the number of caves decreases while the size of the individual caves increases. A large cave near the surface may consist of a few dozen linked chambers, each perhaps a few hundred feet long and a few dozen feet wide. But deeper down are openings in the earth that dwarf any surface dweller’s conception of a cave.


It is not universally true in Faerûn that descending 40 or miles straight down in any spot brings a traveler to Magma. Volcanic activity is extremely variable in the Underdark. Isolated pools of magma seep up almost to the surface in all sorts of places without any other volcanic activity, and in other places deep tunnels and vaults support humanoid settlements at depths where magma should be all that’s present.

Again, planar anomalies, deific intervention, and the powerful Magic of the earth itself are likely to blame. Whatever the cause of these surprising conditions, racing rivers of molten rock, caverns full of brimstone and sulfurous reek, and scalding geysers and hot springs can be found at almost any depth in the Underdark.

Underdark volcanoes aren’t really mountains – they are usually tremendous fissures or magma chambers that can vomit deadly rivers of lava into nearby caverns with little or no warning.

Location - Underdark Volcano
Location – Underdark Volcano



While most of the Underdark is exactly that, some locales possess sources of natural illumination.

Luminescent Growths: Rare forms of lichen, fungi, and moss are bioluminescent. In sufficient quantities, these growths can illuminate large caverns. Most bioluminescent growths are quite dim, so a cavern illuminated by glowing moss or fungi is typically as dark as Starlight, although some particularly bright regions might be equal to Moonlight.

Molten Rock: Areas with exposed pools or streams of magma are illuminated by the dim, ruddy glow of the molten rock. The glare of molten rock is typically equal to Moonlight.

Radiant Crystal: Some rare rocks of the Underdark are naturally radiant, ranging in brightness from starlight to full daylight, although daylight equivalence is quite rare. The great vault of Deep Imaskar is roofed with radiant crystal that is as bright as weak surface daylight.

Reflective Stone: While not naturally luminescent, caverns made partially of reflective stone can be much more easily illuminated by small light sources than normal. Reflective stone quadruples the radius of illumination of any light source brought inside.

Wizard Fire: The rarest and most wondrous of natural illuminations in the Realms Below, Wizard Fire consists of dancing sheets of dim light, like the northern lights of the surface world. Though it is only as bright as starlight, Wizard Fire is weird and beautiful.


TERRAIN: Well-Formed Cave – DISSTANCE: 2d4 x 10′ feet*

TERRAIN: Fungal Forest – DISSTANCE: 3d6 x 10′ feet*

TERRAIN: Gorge or Shaft – DISSTANCE: 6d6 x 10′ feet*

TERRAIN: Vault Floor – DISSTANCE: 6d6 x 10′ feet*

TERRAIN: Rift, Tunnel or Abyss – DISSTANCE: Limit of Sight**

*If one party is carrying light equal to torches or brighter.
**A light source can be seen at a distance equal to 20 times its radius of illumination.


In general, the Underdark is surprisingly well ventilated. Vast subterranean spaces and the rare planar connection to the

Elemental Plane of Air provide plenty of good air for living creatures. However, this is not universally true.


Sealed chambers, isolated caves, and water-isolated caverns have finite supplies of good air. Creatures that don’t breathe need not concern themselves with air quality, but other travelers do.

Any enclosed space that is reasonably airtight can quickly become stale or depleted. In general, a Medium creature depletes about 1,000 cubic feet of air (a 10-foot cube) every 6 hours. A Small creature requires one-half as much air, and a Large creature four times as much. Air becomes stale when it is halfway to depletion.


Regions that are poorly ventilated are not immediately deadly, but the stale air can quickly wear out even the hardiest travelers. A character in stale air must succeed on a Constitution roll each hour (CON +1 per previous check) or become fatigued. A fatigued character must succeed on a saving throw each hour vs Paralyzation (Save +1 per previous check) or become exhausted. An exhausted character takes 1d6 points of non-lethal damage every 15 minutes until the condition is corrected. Characters cannot recover from their fatigue or exhaustion until they reach an area of good air.


Depleted areas can prove deadly in a matter of minutes. See Holding Your Breath, page 238, PHB.


Some areas of the Underdark are plagued by air that is not just stale, but actively Toxic. Often this situation results from geothermal activity in the vicinity of Volcanoes. Sometimes the escaping gases emerge with an audible hiss or a putrid smell, but not always.

Gases tend to concentrate in areas that are isolated in some way from the nearby passages. For example: a passage that dips down sharply and then climbs up again forms a natural, low-lying pocket where deadly fumes can accumulate. An-air-filled passage sealed by water siphons at either end could also concentrate deadly gases.

Irritating Fumes:

These gases cause coughing, stinging of the eyes, dizziness, and similar difficulties. A character exposed to irritating fumes must succeed on a Constitution roll once per round/minute (Constitution, +1 per previous check) or become sickened. Recovery from this condition is not possible until the character leaves the affected area.

Poison Gas:

These gases are deadly. Anyone who ventures into an area of Poison Gas must succeed on a roll vs Poison once per round/minute (Save, +1 per previous check) or take 1d4 points of Constitution damage.

Some poisonous gases also have the characteristics of irritating fumes, so their presence is obvious. Others, however, offer no telltale burning of the throat or stinging in the eyes to warn creatures of the danger. Any creature exposed to poison gas is entitled to a Perception roll to detect the threat before breathing enough to force a saving throw. With a successful check, the creature can retreat before risking any damage. A creature with the scent ability gains a +5 bonus on this check.

Explosive Gas:

Some naturally occurring gases can explode in the presence of open flames. If a burning torch or lit lantern is brought into a pocket of explosive gas, the vapor explodes, dealing 3d6 points of damage to each creature in a 10-foot radius (Save vs Breath Weapon for Half Damage).

An alert Adventurer might notice the danger before causing an explosion, since the open flame often behaves strangely (burning in a different color or exceptionally brightly) right before the explosion. Allow the creature carrying the light a Perception roll to observe the danger before the explosion actually occurs. If an open flame remains in the hazardous area thereafter, there is a 50% chance of an explosion in each subsequent round.


The temperature underground varies depending on the depth and the presence of geothermal features, but the vast majority of the Underdark has a moderate temperature throughout the year. It tends to be on the clammy and chilly side, but typical adventuring garb is sufficient to keep characters warm and dry.


Water is almost always dangerously cold in the Underdark, since it is never warmed by Sunlight. A character who becomes soaked in cold water is much more vulnerable to cold conditions than a dry character. Moderate temperatures are treated as cold temperatures for the purpose of determining whether the soaked character suffers cold damage. This vulnerability remains for 2d4 hours or until the character changes into dry clothes; soaked clothing provides no protection.

A character who remains immersed in cold water for more than 1 round/minute takes 1d6 points of non-lethal damage per additional round/minute of immersion.


Caverns heated by geothermal activity can be quite warm; in fact, hot or severe temperatures are common near volcanoes. Any chamber with large amounts of exposed magma is severely hot, and possibly extremely hot.


Besides its many predatory races and monsters, the Underdark is also home to a variety of natural animals that live out their lives normally below the ground. These animals include (but are not limited to) Bats, Crickets (and Cricket droppings and eggs), Eyeless Cavefish and Crayfish, Springfish, Salamanders, Rats, Flies, Gnats, and Spiders.

Additionally, it is not unusual for creatures in the Upperdark to encounter the occasional Raccoon, Frog, Stray Dog, or even lost Dwarf, Elf, or other representative of a surface race who has accidentally fallen down a deep shaft or well.




Explorers routinely encounter common dungeon hazards such as yellow mold or green slime in the Underdark. In addition, the Realms Below are home to a variety of unique perils and diseases that have laid low more than one Heroic Adventurer.


Many Natural Caves are extremely difficult to navigate. Narrow Fissures, Corkscrew Passages, and low Ceilings are common obstacles in the limestone Caves of the Underdark. The Tunnel Constriction summarises the information related below.

Any creature fighting in a cramped space loses its Dexterity bonus (if any) to Armor Class. Beyond that, the following adjustments apply, depending upon the prevailing conditions.

Narrow OR Low: An area that is smaller horizontally than the creature’s space or smaller vertically than its height falls into this category, so long as the constricted dimension is at least one-half the creature’s space or height, respectively. A creature in such a space moves at one-half its normal speed because of the cramped conditions, and running and charging are impossible. The cramped creature takes a -2 penalty on attack rolls with Light Weapons and a -4 penalty on attack rolls with One-Handed Weapons. It cannot use Two-Handed Weapons at all.

Narrow AND Low: An area that is smaller in both the horizontal and vertical dimensions than the creature’s space falls\ into this category, so long, as each of the constricted dimensions is at least one-half the creature’s space or height, respectively. A creature in these conditions moves at one-quarter normal speed and takes attack penalties equal to twice those above.

Crawl-Navigable: An area less than one-half but at least one-quarter of the creature’s height is crawl-navigable. The creature can move through such a space by falling prone and crawling at a speed of 10′ feet per round, but it must remove Medium and Heavy Armor, backpacks, and other bulky equipment (although these can be dragged along behind). The normal penalties for lying prone apply. The only ranged weapon a creature in a crawl-navigable space can use a Crossbow.

Awkward Space: An awkward space is narrower than narrow, lower than low, or smaller, than crawl-navigable, but not quite a tight squeeze. For example, a fissure 2′ feet wide is less than one-half the space of a Medium creature and is therefore more cramped than a narrow space, but any Medium creature should be able to wriggle through with a little work. A creature in such a space can move 5 feet per round with a Dexterity check. Fighting in an awkward space is possible only with Light Weapons, and the creature takes a -8 penalty on its attack rolls. The only ranged weapon a creature in an awkward space can use is a Crossbow.

Tight Squeeze: A tight squeeze is an area larger, than the creature’s head but smaller than its shoulders. The creature can move 5′ feet with a Dexterity check and it must remove Medium and Heavy Armor, backpacks, and other bulky equipment (although these can be dragged along behind).. Fighting in a tight squeeze is Impossible.


Some weapons are, more suitable for limited space than others. Piercing weapons that jab rather than slice at an opponent are treated as one size category smaller than normal for the purpose of calculating the penalty on attack rolls. Such weapons include the Dagger, Short Sword, Rapier, Spear (any type), Stave, Javelin and Trident, but not the Pick, Scythe, or Polearms.

Attacks with ranged weapons take penalties appropriate to their sizes in narrow or low conditions. Crossbows are the only ranged weapons usable in crawl-navigable and awkward spaces, but attacks made with it still take size-appropriate penalties.


ConstrictionMove Penalty*Attack Penalty Light Weapon**Attack Penalty One-Handed Weapon**Attack Penalty Two-Handed Weapon**Ranged Weapon
Narrow or low1/2 Speed-2-4-8UnuseableAny
Narrow & low1/4 Speed-4-8UnuseableAny
Crawl-navigable10′ Feet Only-4-8UnuseableCrossbow Only
Awkward spaceDex Roll 5′ Feet-8UnuseableUnuseableCrossbow Only
Tight squeezeDex Roll No Armour
5′ Feet

*A creature moving through cramped quarters of any constriction loses its Dexterity bonus (if any) to Armor Class.

**Treat piercing weapons that are jabbed at the target as one size category smaller.

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