Horizon's Underdark

The vast labyrinth of subterranean caverns and tunnels is a place few on the surface are even aware of. Fewer still are those willing to delve into its depths.
Despite this, the Underdark is home to a diverse biome of creatures and cultures. In the northern reaches of Horizon, another world lurks just below the surface of the cold, harsh wilderness. Below are some of the major settlements and regions of this area:
Hiulindar Hiulindar is the largest duergar settlement in the north and is home to well over 4,000 duergar. It was established nearly 2000 years ago by a contingent of duergar who migrated north from their more plentiful settlements beneath southern Kentro. It was built in a natural cavern above a bountiful vein of precious metals. Unlike the outposts they built later, the layout of the city is much more organic (though still rigidly planned).
The center of the city is dominated by an imposing fortress and intimidating siege weapons. Surrounding it are the various smelters, smithies, casters, forgers, and other establishments that drive their society. Aside from the builders, the "outer" sections of the city contain a number of "farmers" who maintain small herds of Underdark livestock (ranging in weirdness from bovine-like mammals called burkag to giant spiders and centipedes) and carefully cultivated fields of fungi.
Min'Muldar The iron mine and subsequent town were built by the duergar approximately 1500 years ago. Like all things dwarven, it was built to last. In true duergar fashion, it is built with a high preference for functionality over form. The streets form a grid pattern, and most of is facilities are dedicated to processing the iron and stone pulled from the mine.
Min'Muldar is the largest outpost for the duergar in this area and was the first established by the city of Hiulindar. As such, a relatively clear path runs between the two and maintained a moderate garrison that was absolutely crushed when the mind flayers attacked utilizing enthralled drow solders.
The Mind Flayer Colonies The colony of mind flayers deep beneath the northern reaches of Horizon is referred to as Tel'aithu (tell-eyeth-oo) in deep speech. It was formed centuries ago by an ulitharid who wished to expand the illithid's influence and reclaim some of the duergar who had ventured north centuries prior.
It was established in secret near to the duergar city of Hiulindar and the drow city of Dur'galad and built by the still-captive-at-the-time Darro (who escaped some time after the colony's completion). For centuries, the illithid built up their strength; slowly and discreetly abducting duergar, drow, quaggoths, kua-tua, grimlocks; anything they could get their tentacles on.
During this time, a new Ulitharid was born into their midst. Tensions quickly rose between it and the elder brain. This new ulitharid was every bit as expansive and aggressive as the original elder brain and then some. It took every opportunity to question the elder brain: why weren't they taking more thralls? Why where they not creating more illithid? They had the numbers, why not outright attack? Why are we not focusing on our true enemies, the gith? Eventually, the ulitharid came to the conclusion that the elder brain was weak. With that, it took half the colony and started a competing one, dubbed Aith'Lida (eyeth-lee-da). However, rather than seeking out to strike out on its own, the ulitharid conspired to take over the original colony with the intention of slowly bringing the entirety of the Underdark under its sway. With a united empire underground, the illithid could once again rise from the depths, defeat the gith, and take their place as the rightful rulers of all sentient life.
Tel'aithu is situated within the perimeter of a yawning chasm, tucked into the rock face in a way that makes it all but imperceptible to anyone not looking for it. A waterfall cascades over the southern end of the pit offering fresh water for the colonies inhabitants. Fungi grows in abundance beneath the waters to provide nourishment for the illithid's thralls. The lair of the elder brain is situated within the cliff face behind the water fall, accessible only through a narrow ledge and a small tunnel.
Aith'Lida, in contrast, is built on a tall "cliff" overlooking the encampment of a sizeable tribe of quaggoth. The lair of the ulitharid is more or less inaccessible to the quaggoth as no ramp or lift leads up to it. Rather, the mind flayers who inhabit it simply levitate up to it. Construction workers (who are still in the process of finishing the lair) are often "floated" up to it, but no unauthorized thralls can accidentally find their way up.
Dur'Galad The drow city of Dur'Galad is situated on an island in the center of a deep underground lake. Its towers of polished obsidian and spires of dark crystal (a dark reflection of surface elf architecture) were constructed over centuries by the dark elves looking to carve out a place for themselves in the unforgiving Underdark. The city itself is well over four millennia old and founded well before the illithid came to the area. It has changed hands between various drow houses dozens, if not hundreds of times, though only a handful of these have been through outward assault. One of the most recent campaigns led to the construction of the fortress Cardhon Firn.
The layout of the city reflects what the creed of the drow has been for thousands of years: leave us the hell alone. Only one small bridge leads to the city gates, whos walls are dark, imposing, and sharp. The main cavern containing the city is surrounded by several smaller caverns containing "towns" where some of the smaller families farm underdark mushrooms and livestock as well as operate mines and "lumber" mills. These towns are called: • Vardarianna - the lumber town that harvests the giant mushrooms and uses their stalks for lumber and construction • Sab'Toven - a mining town where the majority of Dur'Galad's iron comes from • Quarrui - a quarry and stoneworking town that harvests obsidian from a long extinct volcano • Radag-Ivann - a farming town that focuses on livestock and cattle • Ivann-Chwann - a farming town that focuses primarily on agriculture
The city proper is arranged in districts much like a surface city. The districts are arranged in a spiral around the center keep, a magificent fortress carved into a massive stalactite. Unlike other underdark societies like the duergar that are non-materialistic and community minded, the drow actively engage in commerce and have their own currency. Artisans and merchants maintain bustling markets and the various families compete for riches and prestige.
In regards to the illithid civil war, the mind flayers knew they could never take Dur'Galad by force. And so they conspired to take it through subterfuge. The mind flayers lured the drow's matriarch out of the city by seeding rumors of an artifact of Lolth somewhere close by. When she was far enough away, duergar thralls sprung an ambush. The matriarch and her entourage were able to escape and seek refuge in Cardhon Firn. But while she was trapped there as the duergar thralls sieged the ruin, the illithid infiltrated the city and slowly took control of key figures in the drow government. With that, they enthralled the army and have sent them out to fight the ulitharid. It was not a clean takeover, however. The drow army marched through the city, taking every able bodied elf to become another thrall, but many were able to escape and flee south. These are the refugees that have been pouring into Precipice in recent days.
Those that escaped found themselves either retreating deeper into the Underdark by way of the Low Way ruins nearby or skirting the edge of the Lost Canyons before picking up the still functioning Low Ways to the east. The latter hoped to find refuge in Kal'Nerroth after being pursued relentlessly by the agents of the illithid. However, the dwarves turned them away. With return to the Underdark cut off, they had only one option: head to the surface and seek aid from the humans. The dwarves were not entirely heartless and informed the drow that the city of Precipice would be willing to take them in.

Minor Locations Aside from killer monsters and bloodthirsty humanoids, the Underdark is home to a whole host of breathtaking natural phenomena and fascinating ruins.
Ruins of Mel'Undor. Once a major hub for Dwarven trade in the north, this ancient ruin straddles the river Vlokon and has several levels running vertically on each side of the canyon connected by a series of bridges. It is accessible to the north through a massive lift located in the Low Ways and to the east and west again by the dwarves underground roadways.
Ruins of Kal'Kadrin. Another dwarven ruin overlooking the Deep Lake. It was once revered for its smithing and casting districts that produced some of the finest metal works the old dwarven empires had ever seen. Now it is nothing more than a decrepit ruin housing a colony of grimlocks who lurk through the old streets oblivious to the lost history and culture here.
Ruins of Altum. Altum stands as a testament to the tenacity of the Anteceans. Altum was one of several forays the old empire made into establishing a foothold in the Underdark, and what started as a small fortress built around a Foci grew into a small community as more and more researchers moved down there. Its most notable feature was the artificial "sun" installed at the top of its main tower that simulated the day and night cycle of the surface. Interestingly, the sun still works to this day.
When the Great Collapse occurred, Altum was flooded, as the access shaft the Anteceans built was quickly engulfed by the Agate Sea. The entire ruin is now entirely underwater in the depths of the Flooded Caverns. Recently, a small group of triton have moved into the ruins, having been drawn to the strange lights created by the artificial sun.
Flooded Caverns. This series of caves and tunnels was not always underwater. However, when the Great Collapse occurred, millions of tons of water suddenly surged through the access shaft of Altum and submerged a number of square miles of caverns.
Stone Forest. An enormous cavern featuring stalagmites as big around as redwoods and stalactites that loom down from the ceiling taller than most modern towers. The volume and the density of these pillars resemble that of a forest on the surface, hence the name. A multitude of dangerous creatures make their homes here, especially ropers.
Cardhon Firn. A centuries old drow fortress carved out of a massive stalactite tucked into a dead-end cavern. It was originally built as a hidden base for a drow warlord looking to take Dur'Galad. The inverted castle is accessible only by a single stone bridge that spirals around the perimeter of the cavern before stretching out to the structure.
Lolth's Garden. A "lush" cavern containing a mushroom forest where bountiful game and plant life can be found. It is often considered to be the bread basket of Dur'Galad. It is no garden paradise however, as the creatures who make their home there are as liable to kill the hunters are as the hunters are to kill them. Regardless, Lolth's Garden is what drew the drow to this area of the Underdark to begin with.
The Lost Canyons. This series of canyons and hollowed out tunnels form a natural labyrinth that few who enter are able to navigate. The area itself is treacherous enough without the host of wildlife ready to tear apart anyone foolish enough to enter. For this reason, very few of the civilized societies in the Underdark ever venture in here. However, it is rumored that a colony of deep gnomes have made their home here, somehow overcoming the dangers of the canyons.