This is an old post of mine from r/dndbehindthescreen. A few years ago they did an ‘Underdark Month’, and this was the contribution I made to it. This post was originally going to be followed up with another that went into further detail on how to design encounters using the possibilities this post created.
Sadly that second post never came to fruition. I’m instead going to release it here, because I feel there are useful ideas in this concept. Anyway, on with the content.
Life In The Underdark
Firstly a disclaimer, this was an Underdark designed for Tier 3 play. My party entered it at level 11, and it was a large party at that (6-8 members).
The first thing I looked at with designing a reasonable ecology for the Underdark was addressing what I felt to be the biggest issue: how does an advanced civilisation in the Underdark feed its populace? In the Underdark you have Duergar, Drow, possibly Deep Gnomes, and Mind Flayers. Also in my setting is an entire faction of Dwarves locked in an endless war against the Duergar.
My simple solution was that they would do the same as any other surface-dweller civilisation: find high-nutrition crops and breed them selectively. As such, all the Underdark races (with the exception of Mind Flayers) had a diet that consisted mostly of a selectively-bred fungus that fulfilled most of their dietary needs. Using simple water creation spells they were able to develop and refine irrigation systems to the point where they could feasibly farm enough crops to feed a populace, with one advantage being that without the impact of seasons crops could be grown year-round allowing for very reliable harvests and very predictable food inventories. With the Underdark being as dangerous as it is, hunting is effectively out of the question.
Indeed most Underdark civilisations are not masters of their environments in the same way most surface-dwelling civilisations are. Their entire interactions with the wildlife are defensive, at times fighting for their very survival against incursions from the various creatures of the Underdark. Such fights may yield edible meat, but unlike their hyper-reliable agriculture this would be an extremely unreliable source of nutrition. Indeed, the ideal settlement would be built to be completely safe from such incursions, altogether removing this as a potential food supply.
Basically, everyone in the Underdark is vegetarian (or indeed Fungitarian).
Much like our own surface ecology there is a simple chain of plants -> herbivores -> carnivores. For the sake of survival, the majority of these carnivores would likely technically be omnivores, but would eat plants only as an ’emergency food supply’ when prey is lean. This creates two evolutionary arms races. The first is the arms race between plants and herbivores, the other is the arms race between herbivores and carnivores.
The majority of the plan life is fungal. This means large networks of inaccessible roots that are not at risk of being stripped away in the same way the surface fungal caps are. As such, the fungus can rely on never being truly eaten to extinction, and so it does not care for how much of it gets eaten by herbivores.
Symbiotic relationships would also likely begin to exist between plants and herbivores, much as they do on the surface. As the fungal caps are eaten they will release whatever spores they can. These spores will stick to the bodies of the herbivores, who will then move around and passively spread them.
For the herbivorous side, there are two senses that must be amplified in order to be successful. Plants don’t move, so tremorsense and blindsight are not as useful for herbivores as they are for carnivores, but they can be smelled with ease. Smell would be a predominant sense for such creatures.
The second important sense, and one that is also critical in the arms race between herbivores and carnivores, is sight. A large amount of plant life produces a low bioluminescent glow. This benefits the fungus as it attracts the very creatures that it will latch its spores on to, and herbivores use this latent glow to find their sources of food (note that not ALL plants and fungus would have this property, but some will have tapped in to the evolutionary potential of using animal life as part of its reproductive cycle much as flowers rely on birds and bees).
In the realm of carnivores vs herbivores, you have a simple dichotomy. Herbivores move, which means carnivores will want strong tremorsense and blindsight to locate them, while herbivores can also see, meaning carnivores need significant stealth abilities to be able to catch prey. This creates an entire ecology of ambush predators, and it is the abundance and predominance of this type of creature that makes the Underdark so dangerous. It is from here that we get Cave Fishers, Darkmantles and Hook Horrors.
There are also other strategies to avoid detection from herbivores: Camouflage and Flight. Some creatures may disguise themselves as harmless fungus, even emitting a similar bioluminescent glow, but are indeed carnivorous creatures (or even carnivorous plants!). Others may dwell in the larger caverns of the Underdark using flight as a means to go undetected from ground-dwelling herbivores. Cloakers are a classic example of this, with a very mild camouflage to help against the herbivores that decide to look up.
The Net Effect
So to summarise, the Underdark’s danger is a product of its environment. You must have hardy plants, herbivores that are good at finding them, and then carnivores that are good at hunting herbivores. As such, ambush predators will be the most successful carnivores, and this leads directly to the ecological environment that makes the Underdark so dangerous for adventurers. Even those with darkvision lack the tremorsense, blindsight and general awareness to spot a perfectly still, impeccably camouflaged Cave Fisher.
The first time your adventurers roam the Underdark these things should take them by surprise every single time, but as they grow more seasoned and experienced (both the players AND the characters) they will become more and more adept at spotting and avoiding such encounters. Ambush predators follow a simple ethos: If I do not have the element of surprise, I cannot win the fight. An ambush predator will strike hard, fast and unseen. The moment a creature does not go down fast, or pre-empts the would-be assailant, the ambush predator will leave.
I hope you enjoyed this piece. As I mentioned at the start, I’ll be following this up with a piece that shows how you can build specific encounters that lean on this ecology. Included in that will also be the release of a unique creature I created for the campaign I was running at the time of this post: the Cloaking Spider.
I’m really excited to release this stuff after so long in the vault.