So this is a direct follow-up to This piece that discussed the ecology of the Underdark. The main takeaway from it is that the Underdark as an environment favours ambush predators as the dominant species.
This piece will discuss how that informs encounter design when running adventures in the Underdark.
To Catch A Predator
One of the key things I highlighted in the last piece was that in the Underdark pretty much every single creature is going to be more adapted to living there than the party. This means they should inherently be at a disadvantage in most situations (at least to begin with, but more on that later). Even characters with Darkvision won’t be able to see as keenly as the native fauna. This means firstly that the party is far more likely to be spotted by a creature than they are to spot the creature.
Many combats will start with the enemies having the element of surprise (as in, the party will literally have the ‘Surprised’ condition). This gives the creatures a huge alpha-strike on the first round of combat, and the rest of the fight will be the party ‘catching up’ or potentially trying to stabilise before fighting back. This is something that should be accounted for in your design, and it is wise to ensure creatures aren’t too dangerous compared to what your party can competently best in the surface.
Secondly, ambush predators don’t like a protracted fight. They will attack with the element of surprise, take their prey, then get out. And they should be good at getting out. This means two things:
– The alpha-strike turn is even more dangerous, as a downed party member will likely be abducted and taken to somewhere the creature can safely eat.
– If the tide of battle turns, the enemies will flee immediately.
Both of these things put together will make your combats incredibly immersive. Even if an enemy is strictly weaker than the party, their sheer suitability to the environment is going to make the players feel like they are constantly on the back foot.
In my opinion the Underdark should be deadly, and it should be deadly in a very different way to the surface world.
Assuming your party has access to resurrection magic, they may be used to death being a trivial thing. The Underdark flips that entirely on its head. Remember what I said above about a creature taking a downed party member to its nest to safely feed? Now if the party wants to resurrect their fallen comrade they’re going to have to track something that is incredibly good at not being tracked, then fight it again in its favoured environment, and only then will they be able to resurrect their companion (if they haven’t been eaten already).
Again, the deadliness is a natural product of the environment rather than just the encounter itself. It’s reminiscent of the Tucker’s Kobolds approach. Combats aren’t deadly because enemies have a bucket of hit points and hit like a freight train, they’re deadly because the enemies are efficient and use the environment to their advantage.
Another side note to add to the ‘deadliness’ factor, in your Underdark diamonds may well be scarce compared to on the surface world. Indeed, given that settlements have to be somewhat self-sufficient and trade between them is limited, if a settlement isn’t mining its own diamonds then they just straight-up won’t have any for sale.
This is where I think some creative liberties need to be taken to really amp up the Underdark’s danger.
I like to adjust monster statblocks in the Underdark to give them abilities that reflect how they survive in the environment. A predator will have a higher stealth skill than a surface counterpart. Most creatures will have some sort of way of reliably escaping from fights they can’t win. They will have abilities that make disabling prey easy for them.
An example of how I implemented this personally was I gave Cloakers the ability to grapple a creature and fly away with it. Suddenly something swoops down from above, clamps on to a party member’s head, then picks it up and starts flying away. Now the party need to down the creature, but when they do the party member will take fall damage. It’s a no-win situation, and regardless of the outcome the party is probably going to have to blow a Revivify on the unlucky soul that got picked up.
Providing similar tweaks to other monsters, such as a one-use ability that lets them disengage and flee, or make a stealth check with advantage in dim light to hide, allows them to more closely align with the fantasy of ‘hit-and-run ambush predators’.
The Cloaking Spider
In the last part I discussed a homebrewed monster of mine that I added to the Underdark. I’m going to talk about that monster now.
The cloaking spider has the innate ability to turn invisible at-will. It has a bite that injects a petrifying toxin, and it feeds by breaking down the minerals in a petrified creature with its highly acidic mandibles.
Pretty simple from an ecological standpoint. This is a creature that takes a target by surprise, petrifies it, then feeds on it whenever it is safe to do so. This means they also don’t have to eat the creature right away. They could comfortably petrify one animal in a herd, then return later when the herd has moved on to consume the petrified creature.
To add a beautiful bit of elegant design, the setting also had a potent anti-petrification elixir, but it requires Cloaking Spider venom to manufacture. This is similar to the old feature of Basilisks where you could cover a petrified creature in a Basilisk’s bodily fluid (usually blood) to un-petrify it.
So now you have a creature that more or less has a ‘save or suck’ feature, but with a reversible effect. However, that effect requires you to get the better of a creature that just got the better of you. This whole design feeds into a brilliant gameplay loop.
The first time the party gets attacked by a Cloaking Spider they’ll likely get absolutely trounced. The party won’t see or hear them coming, and suddenly this huge spider is upon them, someone gets bit, people take damage, there’s a good chance someone will get petrified, then the spider vanishes before it can be killed.
Every single part of that is preventable and counterable though. Now that the party has fought them once they know what they’re up against. If they’re now having to hunt a Cloaking Spider to harvest venom for the anti-petrification elixir then they can do so a little bit more aware of what they’re up against. The Wizard will know to prepare See Invisibility, the Ranger will know to look for places where rock has been burned away by acid, the Fighter will know to grapple and restrain the spider rather than smack it with a sword. Even if the Cloaking Spider gets the drop on the party the next time they encounter it, they are more well equipped to recover quickly and gain the upper-hand in the fight.
Before long the party will be one step ahead of the Cloaking Spiders they encounter, spotting trace of them and popping spells that let them detect them well in advance. The spider will no longer be able to gain the element of surprise, and without the element of surprise this ambush predator will likely leave the party well enough alone.
You can also mechanically represent the party’s growing familiarity with these creatures by lowering the DCs made to track and spot them the more the party encounters them. The Ranger now knowing to look for traces of corroded rock might mean the Survival DC to track the creatures is 15 instead of 20, for example.
To give a second example, after being absolutely slapped by a Cloaker once the party may adapt to having the Rogue carry a net that they are always ready to throw, or perhaps the Ranger buys a scroll of Ensnaring Strike that they can use in a pinch.
Masters Of Their Environment
By the time the party is done in the Underdark they will know every creature, how it hunts, how to track it and how to beat it. They will be prepared against all threats, and the creatures that brought them low the first time they stepped out into the untamed caverns of the Underdark are now a triviality.
But the threat isn’t over, for in the Underdark there lurks other threats who are themselves masters of their environment. Duergar, Drow or the dreaded Illithids will present a threat above and beyond what a simple ambush predator could present. Just as the party gets on top of simply navigating the environment they are suddenly faced with threats from sadistic masters of magic and torture, or mind-bending psionic horrors.
The Underdark is a dangerous place. It is hostile to its very core. Maybe the party should never have come at all…
So there you have it. Encounters in the Underdark absolutely skew toward deadliness, which also means mastery of the environment is rewarded far more greatly than it often is on the surface. With a few simple tweaks, or even just changes in how you handle creature behaviour, you can make a deeply immersive Underdark that truly lives up to its reputation as a dangerous place to venture.