If you are thinking of sending your party into the depths of the earth, (or if they just happened to wind up there) there are a whole new set of concerns for you as the Dungeon Master.
The Underdark is like an alien world lying just below the surface, if your party is going there, you are going to want to be sure you aren’t making these mistakes.
How do you run an adventure in the Underdark? You need to make sure you are considering these features of the Underdark:
- Civilizations and Communities
- Gods and goddesses
- Unexpected Refugees
Each of these features has an impact on an Underdark adventure. Keep reading for full explanations on what Dungeon Master’s often miss in each of these key areas.
A Whole New World
The ecosystems of the Underdark are vastly different from that of the surface world. Because there is no sun, there are no plants that perform photosynthesis and, therefore, no typical plant-eaters. This means the Underdark ecosystems mostly consists of fungi and carnivorous creatures (which explains why all the monsters that inhabit the Underdark have teeth and want to eat you.)
Why does this matter? Well, different ecosystems mean that your Druid that normally could tell if something is poisonous or not, due to their high nature check, should have disadvantage on nature checks while in the Underdark (unless they have spent time there before) because of how different it is from the surface world.
This also means the party could be exposed to a whole new world of diseases and poisons that Underdark creatures have immunity to, but surface dwellers do not. Clouds of poisonous fungi spores that weaken the surface adventurers have no effect on the creatures they fight. Or perhaps a party member develops a persistent cough caused by the perpetually damp air.
There are a number of ways you could play this out. Have some fun with it, just don’t forget, this ecosystem is, most likely, completely foreign to your party and that should come with some dangers.
While not necessarily common, some caverns are large enough to have their own weather systems (for real!). Rain is not uncommon in such caverns, and while it isn’t necessary, it could be a fun twist for flavoring that many DM don’t even consider.
Underground terrain is vastly different from surface world terrain, and many DMs forget this when they send their party underground. Here are some features of underground terrain which you need to remember when playing out encounters or descriptions of areas.
You are underground, which means there is always a ceiling to be mindful of. Whether that ceiling is hundreds of feet high, or so low the party must crawl through a space, always beware of the ceiling. Having a ceiling also gives a whole new area to hide creatures or clues, so make sure not to neglect it.
There is always a limited amount of space underground. You need to make sure you are clear with how large caverns, tunnels and other areas are. The amount of room available makes a difference for spells and movement, so to avoid confusion and frustration, always be clear. If you aren’t sure how big caverns should be, you can roll a d20 or a d100 (depending on how large of an option you want) for the length and height of the room (in feet), then add some features, like crumbling ruins or stalagmites, to it.
The Underdark is filled with water. From vast lakes, to subterranean rivers, most of the water on Earth finds it way underground. Including rivers and lakes into your Underdark adventure adds elements of surprise and wonder. What will your adventurers do when they face a vast, endless, inky lake? Wouldn’t it be fun to find out?
Most Underdark rooms and tunnels are carved through rock. Rock does not absorb sound, meaning that even the slightest whisper would travel a good distance. Many DMs forget this aspect of the Underdark when creating their adventures. If your players get into combat, or have a particularly noisy dwarf in the party, you can bet the sound will be heard for miles, alerting other enemies and creatures in the area.
The primary way the party will be traveling through the Underdark is via tunnels. There are lots of creatures that make tunnels in the Underdark. Purple Wurms and Kobolds just to name a few. But tunnels don’t always lead where you think they should. They can double back on themselves, open into a sheer drop, or simply come to an end. Tunnels made by creatures often crisscross each other, creating a labyrinth. Tunnels can also be various sizes. Purple Wurms makes tunnels nearly 10 feet in diameter, while kobold tunnels are only 5 feet. Use various types of tunnels in confusing ways to get your party turned around, making your Underdark adventure more challenging.
Water isn’t the only element found underground. Lake and rivers of lava are also a common sight if adventures go too far down. These can create problems all their own as the party tries to decide how to cross them.
Diamonds, gold, emeralds and other precious metals and stones are all found underground. While I am not saying these should just be littered about, rolling a percentile dice to see if your party stumbles across a crystal cavern or a gold vein is fun. Especially if you know they didn’t bring a pick axe…
Light Sources and the Underdark
It is called the Underdark for a reason. And while most DMs don’t forget that part and make sure the party has a light source, they do tend to forget how that light source might affect the creatures of the Underdark.
If the party has cast the Daylight spell, or some other light spell, if the spell specifically says it creates bright light then most creatures from the Underdark are going to have disadvantage on attacks while that light is on. The party is also going to draw a lot of attention, and not positive attention. As long as a light is on, the party should all automatically fail stealth checks.
This is a homebrew rule, but I think it makes sense. You are bringing a light source into a completely lightless area. How are you going to hide that? Even the tiniest bit of light can be seen from a LONG way away in a cave. If you find this rule too harsh, then give the party disadvantage on stealth checks.
You also need to decide if the light will draw creatures to it, or chase them away. This will depend mostly on the creature, but more vicious creatures would likely try to destroy the source of light while more timid ones might run.
There are a vast array of Underdark creatures and you should be using them! Many DMs get stuck on bats and Mind Flayers, but there are some crazy cool creatures you can use! Here is a quick list of my favorites from the Basic Rules, which are available for free online. The list is organized by Challenge Rating, lowest to highest.
Be sure to check out Volo’s Guide to Monsters and Mordenkainen’s Tome of Foes if you are looking for even more Underdark Creatures! You see a description of the books and purchase them on my DM Must Haves page.
Amorphous. The pudding can move through a space as narrow as 1 inch wide without squeezing.
Corrosive Form. A creature that touches the pudding or hits it with a melee attack while within 5 feet of it takes 4 (1d8) acid damage. Any nonmagical weapon made of metal or wood that hits the pudding corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed.
Nonmagical ammunition made of metal or wood that hits the pudding is destroyed after dealing damage.
The pudding can eat through 2-inch-thick, nonmagical wood or metal in 1 round.
Spider Climb. The pudding can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage plus 18 (4d8) acid damage. In addition, nonmagical armor worn by the target is partly dissolved and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.
Split. When a pudding that is Medium or larger is subjected to lightning or slashing damage, it splits into two new puddings if it has at least 10 hit points. Each new pudding has hit points equal to half the original pudding’s, rounded down. New puddings are one size smaller than the original pudding.
Damage Transfer. While attached to a creature, the cloaker takes only half the damage dealt to it (rounded down), and that creature takes the other half.
False Appearance. While the cloaker remains motionless without its underside exposed, it is indistinguishable from a dark leather cloak.
Light Sensitivity. While in bright light, the cloaker has disadvantage on attack rolls and Wisdom (Perception) checks that rely on sight.
Multiattack. The cloaker makes two attacks: one with its bite and one with its tail.
Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 10 (2d6 + 3) piercing damage, and if the target is Large or smaller, the cloaker attaches to it. If the cloaker has advantage against the target, the cloaker attaches to the target’s head, and the target is blinded and unable to breathe while the cloaker is attached. While attached, the cloaker can make this attack only against the target and has advantage on the attack roll. The cloaker can detach itself by spending 5 feet of its movement. A creature, including the target, can take its action to detach the cloaker by succeeding on a DC 16 Strength check.
Tail. Melee Weapon Attack: +6 to hit, reach 10 ft., one creature. Hit: 7 (1d8 + 3) slashing damage.
Moan. Each creature within 60 feet of the cloaker that can hear its moan and that isn’t an aberration must succeed on a DC 13 Wisdom saving throw or become frightened until the end of the cloaker’s next turn. If a creature’s saving throw is successful, the creature is immune to the cloaker’s moan for the next 24 hours.
Phantasms (Recharges after a Short or Long Rest). The cloaker magically creates three illusory duplicates of itself if it isn’t in bright light. The duplicates move with it and mimic its actions, shifting position so as to make it impossible to track which cloaker is the real one. If the cloaker is ever in an area of bright light, the duplicates disappear.
Whenever any creature targets the cloaker with an attack or a harmful spell while a duplicate remains, that creature rolls randomly to determine whether it targets the cloaker or one of the duplicates. A creature is unaffected by this magical effect if it can’t see or if it relies on senses other than sight.
A duplicate has the cloaker’s AC and uses its saving throws. If an attack hits a duplicate, or if a duplicate fails a saving throw against an effect that deals damage, the duplicate disappears.
Huge Monstrosity, neutral evil
Challenge Rating: 11 (7,200 XP) (see full stats at DndBeyond)Actions
Multiattack. The behir makes two attacks: one with its bite and one to constrict.
Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) piercing damage.
Constrict. Melee Weapon Attack: +10 to hit, reach 5 ft., one Large or smaller creature. Hit: 17 (2d10 + 6) bludgeoning damage plus 17 (2d10 + 6) slashing damage. The target is grappled (escape DC 16) if the behir isn’t already constricting a creature, and the target is restrained until this grapple ends.
Lightning Breath (Recharge 5–6). The behir exhales a line of lightning that is 20 feet long and 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.
Swallow. The behir makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the behir, and it takes 21 (6d6) acid damage at the start of each of the behir’s turns. A behir can have only one creature swallowed at a time.
If the behir takes 30 damage or more on a single turn from the swallowed creature, the behir must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the behir. If the behir dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.
Large aberration, lawful evil
Challenge Rating: 10 (5,900 XP) See full monster stats and lair actions at DnDBeyond.Aboleths lair in subterranean lakes or the rocky depths of the ocean, often surrounded by the ruins of an ancient, fallen aboleth city. An aboleth spends most of its existence underwater, surfacing occasionally to treat with visitors or deranged worshipers. This terrifying creature can cast spells, such as charm person to enslave creatures it comes across.
Amphibious. The aboleth can breathe air and water.
Mucous Cloud. While underwater, the aboleth is surrounded by transformative mucus. A creature that touches the aboleth or that hits it with a melee attack while within 5 feet of it must make a DC 14 Constitution saving throw. On a failure, the creature is diseased for 1d4 hours. The diseased creature can breathe only underwater.
Probing Telepathy. If a creature communicates telepathically with the aboleth, the aboleth learns the creature’s greatest desires if the aboleth can see the creature.
Multiattack. The aboleth makes three tentacle attacks.
Tentacle. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 14 Constitution saving throw or become diseased. The disease has no effect for 1 minute and can be removed by any magic that cures disease. After 1 minute, the diseased creature’s skin becomes translucent and slimy, the creature can’t regain hit points unless it is underwater, and the disease can be removed only by heal or another disease-curing spell of 6th level or higher. When the creature is outside a body of water, it takes 6 (1d12) acid damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.
Tail. Melee Weapon Attack: +9 to hit, reach 10 ft. one target. Hit: 15 (3d6 + 5) bludgeoning damage.
Enslave (3/Day). The aboleth targets one creature it can see within 30 feet of it. The target must succeed on a DC 14 Wisdom saving throw or be magically charmed by the aboleth until the aboleth dies or until it is on a different plane of existence from the target. The charmed target is under the aboleth’s control and can’t take reactions, and the aboleth and the target can communicate telepathically with each other over any distance.
Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from the aboleth.
The aboleth can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The aboleth regains spent legendary actions at the start of its turn.
Detect. The aboleth makes a Wisdom (Perception) check.
Tail Swipe. The aboleth makes one tail attack.
Psychic Drain (Costs 2 Actions). One creature charmed by the aboleth takes 10 (3d6) psychic damage, and the aboleth regains hit points equal to the damage the creature takes.
Just because it is underground doesn’t mean there isn’t culture. There are whole cities and ports underground about which the surface dwellers have only heard rumors. The Drow are not a rag-tag group of barbarians, and while the Mind Flayer eat brains, they certainly are not devoid of intelligence. DMs often make the mistake of making their Underdark adventures too rough and rustic when their are sophisticated options as well.
While I could not possibly cover all the intricacies of Drow civilization (see the exhaustive Drizzt series), suffice it to say there most certainly is one.
Drow have a very structured society with Matron Mothers at the top, leading families, and constantly fighting for power. These Matron’s are the centers of vast webs of conspiracies and plans to gain Lolth’s (the Spider Queen’s) favor and sabotage other families.
The Drow build incredible cities underground and guard them using spiders, magic, and poisoned weapons. Though they do not easily trust outsiders, they recognize when they may be of use in their traitorous plans.
Unlike many civilizations in the Underdark, Myconids are a peaceful race. They are welcoming of visitors and very hospitable. These humanoid fungi have some unique past times, often enjoying a group hallucination called “the meld.” If you want to know more about these psychedelic little dudes, check out my post on 5 Monsters to Encourage Role Play
Mind Flayer Civilizations
Mind Flayers, while dark and twisted, still have social expectations and strict societal rules. All Mind Flayers are connected to the Elder Brain, which, in turn, connects all Mind Flayers to each other.
If a Mind Flayer violates a societal rule, their connection is severed and they are shunned from the community. This can lead to groups of Mind Flayers living on the outskirts of Mind Flayer civilizations, surviving as best they can.
Mind Flayers enjoy enslaving other races with their psychic abilities. Some notable examples include the Kuo Toa and the Duergar, or Gray Dwaves. The Mind Flayer mind control caused both of these races to go insane, which persists to this day.
Mind Flayers also like to experiment. Some of their experiments include intellect devourers and Oblexes. While they like to experiment, Mind Flayer never deal with magic. They hate magic intensely and any Mind Flayer that practices it is immediately cast out.
Gods of the Underdark
Two of the most common gods of the Underdark are Zuggtmoy and Lolth. You can also see a list of other dark domain gods at the Forgotten Realm Fandom here.
Zuggtmoy: Demon Queen of Fungi
This truly alien goddess seeks to replace all life with fungi. She is known to bring followers to herself with promises of power if they ingest the mushrooms she gives them. The spores of these mushrooms then take control of the person and slowly force them to do the bidding of the Demon Queen. If the party is wandering through the Underdark, they better watch out for the Queen of Fungi.
Lolth: Demon Queen of Spiders
Worshiped by the Drow, Lolth is a betrayer god who delights in falsehood and treason. She encourages deception in her followers and rewards those who are most devious in their plots. But Lolth’s blessing is fickle. One day she bestows power, the next, for no reason, she will take it away.
All of Lolth’s servants are women. She has no use for men and Drow culture reflects this. Drow men are useful only as warriors, the women rule the houses and tend to political affairs. The Matron Mothers are Lolth’s most powerful priestesses, to whom Lolth grants extraordinary powers, though not always consistently.
Refuge for the Radical
While the surface world may scoff or protest questionable magical practices, the Underdark, while dangerous, is considerably less concerned with such things. This makes it the ideal refuge for the more, let’s say, fanatical of mages. DMs often forget this fact when planning underground adventures. It is entirely possible the group will find something far more disturbing in the tunnels of the Underdark than a Aboleth.
The Underdark provides the perfect hiding place for a necromancer collecting bodies. Perhaps he is building an army of undead to conquer the surface world, or perhaps she is simply looking for a quiet place to complete her research undisturbed. Whether it is creating a new disease or simply an unhealthy fascination with death, necromancers are at home in the Underdark.
Shunned by the people of the surface world, or perhaps chased away, Mad Mages continue their pursuits in the safety of darkness. Building ever more elaborate mazes to protect an ancient item that made them insane in the first place, or hiding from real or imagined monsters, mad mages often find their way to the Underdark. Just ask the folks at the Yawning Portal.
Some cults that cannot find safety above may seek shelter below ground until their forces grow strong enough to fulfill the will of their master. One example might be the Cult of Orcus, the Demon Lord of Death. Such cult members are not often welcome by surface dwellers, and such a cult might grow its numbers underground while waiting for the right moment to strike.
Recommended Gear for Exploring the Underdark
If your party is planning to go into the Underdark, there are a few things they should have:
- Rope (over 200 feet)
- Grappling hooks
- Rope Ladder
- Food or a way to make it
- Light source
- Fuel (for campfires or lanterns if that is the light source.)
- Pickaxe, shovel, or means of digging
Neither you nor your players will ever be fully prepared for the adventures that await you in the Underdark. This alien and beautiful world is home to incredible and dangerous creatures, breathtaking views, and deadly terrain. If your party is brave enough to dive into the depth of the earth, then this guide will make sure you are prepared to greet them properly.
Until next time,
May your game have advantage, my friends!