Underdark

Below the surface, far past the mines of the dwarves and deeper still than most seas, lies the terrible caverns of the Underdark. A place of brutal beasts and denizens who will do whatever it takes to survive in these inhospitable lands. These are the lands of the Underdark, a place so cruel and cold that should one stumble upon it unprepared they will stand little chance at survival, even with the most prepared falling victim to the savagery of the beasts below or the cunning of the races that dwell within it. Carved deep within the world before the rise of the first humanoids and inhabited by creatures that would make a surface dweller freeze in place at the mere sight of them.

Biomes

The layers of the Underdark all vary in their ecology but all share some similarities. Among the most common of biomes come the caverns, sprawling throughout the entirety of the Underdark and littering almost any direction one can look. Large stone paths stretch across the caverns and make natural ways between them and to the varying areas within and while some are natural there are others that seem to  be man-made or crafted by some other means.

The plant life of the Underdark is scarce and mostly populated by fungi though there are some rare plants that do grow in the darkness that are valued by dwellers in the darkness for their poisonous properties. Among them come the lethal plants that make up the drow poison that is often used to aid their warriors in fending off the denizens of the Underdark.

Aside from the mushrooms and caverns there are plenty of reservoirs of water that seem to pool in the deeper chasms of the layers. Some interconnect and seem to endlessly delve while others tend to just be pools of water gathered from the dripping stalactites from above. Near the deeper reservoirs one is likely to find the Kuo-Toa who often set up their villages by the water and keep to themselves.

Of all the biomes the least explored is the waters beneath the Depths. No one has ever ventured so far and lived to tell the tale, or at least ever come back to inform anyone about it. Rumors say that one who sinks too deep into those waters will find themselves in the infinite abyss where light of all types is forever extinguished, left with nothing more than their thoughts and the gnawing blindness all around them.

The Upper Dark

The upper dark is the layer closest to the surface, far deeper than any normal mine or surface body of water save for the Elmham Trench and most oceans deepest reaches. Entrances to the underdark mostly lead to this layer with the very rare exception that one of the tunnels leads to the Deep Dark or worse.

While the Underdark is a dangerous place the first layer is one that has mostly been conquered by the drow, duergar, and deep gnomes. These three races are seen commonly in the Upper Dark, cities built into the caverns and races living mostly in peace with one another. Aside from the cities one might see them prowling the caverns, hiding in the shadows and darkness as they hunt whatever quarry has managed to sneak into their territory. Travelers of the Underdark may also come across the Quaggoth, the once proud rulers of the savage lands reduced to nothing more than nomads of the dark who struggle for survival in the place they once called their own.  Passage between each city is treacherous though as the Underdark is filled with heinous creatures that would make almost any mortal tremble with fear. Horrifying monsters stalk the dank caverns as much as the dark elves or Quaggoth’s do and make travel a difficult trial compared to the safe and warm roads of the surface.

While the Upper Dark is mostly caverns and pathways there are obvious signs of man-made tunnels, mining expeditions run by the Duergar leaving obvious signs that they had been there with their efficient and almost inhuman methods. Abandoned mines are littered all around, whether abandoned because of creatures invading or from the resources running dry? No one knows until they foolishly step foot inside. Even in the darkness of the caverns there are some sources of light, among these are the luminescent mushrooms that seem to litter the more warm and damp areas of the Underdark, another source is the more rare occurrence of glowing stalactites and stalagmites. Despite the best efforts of scholars and researchers no one has figured out quite why these random stones glow at all, let alone how they do it and yet they are a sign of comfort to those not used to the utter darkness of the world below.

The Deep Dark

While the Upper Dark is mostly ruled by the more civil races of the Underdark, the Deep Dark is home to the more hostile denizens. The once abundant sources of luminescent mushrooms seem to diminish the further down one goes and the rocks no longer glow to offer the safety of light to wary travelers. In fact light seems to almost be taboo here as the source is more akin to a beacon for death, those lighting such sources often finding themselves assailed by death itself in the hostile lands.

Monsters dominate the Deep Dark and appear frequently within the layer, groups of hook horrors, bulettes and many other creatures prowl around in search of their next meals with a lethal efficiency that leave even the more skilled adventurers either gravely wounded or snapped up as lunch or dinner. Despite the grave dangers of the Deep Dark the duergar and Drow frequent it in search of rare minerals and resources that are scarcely found within the Upper Dark. A wandering adventurer may at times come across the Drow battling off some heinous creature while defending their Duergar allies, keeping them safe as best they can.

Should one find themselves unlucky they might find a wandering mind flayer. Most people pray they are part of an Elder Brain’s colony and that their quota has been met, but even luck cannot save one from a rogue mind flayer that has their own ambitions in the world with eyes on a fresh meal.

Despite the dangers of the Deep Dark there are many amazing wonders hidden within, whether it be rare resources or simply the carcasses of such monsters themselves. Miners are able to find abundant sources of Mithril and Adamantium down in the Deep Dark and will risk their lives and more to get their hands on it. 

The Depths

The Depths are the most lethal of the layers, ruled over by the most heinous of abominations to grace all of Embris. Aboleths and Mind Flayers are practically the rulers of this inhospitable land and while they may welcome trespassers it is not for pleasant reasons. One would be loath to fall into the clutches of the mind flayers and be brought back to their Elder Brain leader while others may fall prey to a lone aboleth who happened to catch them on their own.

Climate

With the Underdark being a unique environment compared to the surface there are no seasons that govern the climate in any given location or layer, rather the temperature is mostly dependent on how tight or open a location is or its proximity to a water source. A person exploring the wider caverns of the Underdark might find themselves growing cold as the air is free to flow without restriction and often does not have anything to warm it up while the more constricted tunnels or mines will generally be much warmer and cozy. Of course there are exceptions to the general rules of such, especially when one considers the civilization of the Drow and their allies.

The waters of the Underdark tend to be cold and chill one to the bone, a frosty dip for any not already inclined to such temperatures and this theme extends to the areas around them as they chill the air and cool off the rest of the environments around them. Not only that but the air tends to be damp and thick compared to the otherwise clear or fungal infested air of the Underdark.