Káto Geographic Location in Isekai | World Anvil

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The Underdark is a vast expanse of tunnels, cave systems, underground lakes, and subterranean rivers that appear to have no end. If the lands of Isekai that the sun readily touches are massive then the world hidden away is unfathomable in size and scope. The Underdark is a place of mystery, gloom, violence, and constant change that few races have adapted to embrace wholly and the idea of a safe place is laughable as the only way to seek safety is to return to the surface. Those that make their home in the Underdark are strong, having adapted to a world where survival is based upon your ability to fight against monsters that defy explanation or to hide and watch the carnage that they leave behind. No amount of preparation or arcane knowledge is enough to guarantee the safety of a traveler because the smaller tunnels and covers will often shift to close passages or to open new paths that should have remained closed.  
Walk quietly and douse your lights. There are things here that can see through solid stone and feel your ragged breath as you take it. None of them are friendly, all of them are hungry.
— Kithkin, the Drow Gloomstalker
  The true depth of the Underdark is unknown and most assume that it extends to the true bottom of the world where the fabled Abyss Ocean hides the atrocities that the god Caspian drove from the upper seas. Because of this, there are waterways and dark seas that exist far beneath the surface but their true depth is unknown and it is impossible to say where they connect to the vast oceans of Isekai without diving in and charting the paths through experience. For the adventurer that wants to find new or forgotten paths, they might think that the Underdark is a grand place of opportunity but the reality is that it is a graveyard for the inexperienced. Paths can suddenly shift and collapse, the routes between Underdark provinces can collapse from natural occurrences and ancient creatures that were chased deep underground by the Huntress search for their chance at an easy meal or a way back to the surface.   The underground paths and caverns experience no change in seasons, the day is not known from the night and there are few sources of light that occur naturally. The gems or plants that can create their own sources of light are highly coveted and communities often form around them while the creatures that can produce their own light find that it makes an effective lure for the sake of hunting adventures. Concepts of time change for creatures that live below ground, some have adapted to measure time by the formation of crystals or the life cycle of fungus while others have lost touch with that aspect of reality and sleep only an hour or less for each day on the surface. While the dangers of the Underdark are many, the soundness of one's mind is a forgotten danger to many, for those that lose their way in the winding, endless caverns may eventually forget the color of the sky or the warmth of the sun as their mind gradually begins to lose touch with the world that they once knew.   Those that seek to retain their sanity bind themselves together in grand cities or interconnected kingdoms. The grand mines and halls of the Dwarves reach only so far into the Underdark while the dwelling places of the Duergar and Drow can be found unimaginably far below the surface. These often exist as stand-alone organizations where the more civilized Underdark denizens will trade, rest and fight for their territory while the zealous will crusade farther into the depths in search of ancient evils to exterminate. There are also Dragons that seek out the concealment offered by the Underdark and hide their great treasure hoards deep underground, their presence is often considered to be fearsome and terrible to the point that even a Metallic Dragon that lives deep underground is not considered an ally if you trespass into their caverns. Since the Eldritch War, there has also been an increased presence of otherworldly creatures and unspeakable horrors that do not originate from Isekai that have many eyes, limbs, bodies that do not appear to obey the natural laws and physics established in knowledge and some of them wield fearsome psychic abilities.     With these dangers, there is a sense of dread and misfortune that hangs about the Underdark and is generally understood in any culture that has a known Underdark path whether temporary or established. However, there are many things that would tempt adventures to venture here aside from exploration and glory. Precious metals, jewels the size of which would cause a Dragon to covet your wealth, relics of ancient empires and earlier ages when the gods walked freely among mortals. Many of the secrets of the world are locked away in the Underdark, a place that while in constant motion has an uncanny knack for preserving and remembering things that the Dragons do not know and that the Sphinxes will not talk of. It has been said that a civilization of the Ancient Fey once existed that had gained complete knowledge, that understood the entirety of the world and the expanses of all the tunnels and oceans within it. This civilization journeyed to the very bottom of the world and took with them magic and knowledge and secrets that were wiped from history for reasons that are unknown even in legend.


The Underdark is a massive, underground network of tunnels and caverns and waterways that is divided into 6 major provinces that are recognized by most experts. These regions reside primarily under the Northern and Southern Continents of Isekai though it is not uncommon to find a new entrance to the Underdark on an island or even on the seafloor, these are often temporary Underdark paths that may stay open for a year or a week depending on several factors. Provinces of the Underdark are an important distinction because there are few known ways to move between most provinces by using tunnels and waterways, they are self-contained networks that spiral endlessly down into the heart of Isekai. When a path exists to move between provinces it is usually kept secret and coveted by a powerful faction or an Adamantine Dragon, the exception would be the Great Tunnel which spans three provinces on the Northern Continent.  
These provinces are called: Dytikós, Kéntro, Anatolikós, Notiodytikós, Alithinós Nótos, and Notioanatolikós. Smaller regions may exist by a variety of local names, it is not uncommon for new paths to open deep in the Underdark itself which lead to previously sealed off areas that could contain anything from forgotten civilizations, ancient ruins, or more commonly a way that leads ever deeper underground. The paths themselves often have no owners but are fought over by different factions, Orcs, Goblins, Trolls, Kobolds, Drow, Dwarves, Duergar, and creatures that have no other description than horrors. Ownership is held over water sources, large caverns that house towns or entire cities, or hunting grounds that remain stable. Reputation is just as important as fighting power when keeping ownership of these areas because there is little in the way of law outside of an Underdark city and inside of one the master of the city is unquestioned. The cities of the Underdark are far away from other civilizations, often holding little regard for the opinions of surface dwellers nor the likelihood of powerful figures from the surface bothering to come down into the perpetual darkness.

Quick Facts: Geography

Lowest Point: Unknown
Highest Point: Located in the "Metallic Council"
Subterranean Oceans: 13 large oceans known
Fresh Water: Plentiful and High Quality
Illumination: Dim to Pitch Black
Average Temperature: 65 Degrees
Average Tunnel Clearance: 5-10 feet tall
Average Tunnel Width: 2-7 feet wide
Average Tunnel Visibility: 10 feet ahead
  The majority of the Underdark's landscape are tunnels and small caverns, often spiderweb-like in appearance as some have been formed naturally throughout the course of time and others have been dug by creatures and sentient races alike. These tunnels are usually warm, with little in the way of natural light when they are empty. Most caverns and tunnels have a mellow warmth that is deceptively welcoming, the temperature usually dips when near a large abyss or when approaching a body of water. The rivers in the Underdark are small as a rule, with few exceptions being more than ten feet across but the lakes are always deeper than anyone would assume. What looks like a small lake that would house an ample supply of fish will go down hundreds of feet and connect itself to the ocean by way of underwater passages. The most unnerving part of the Underdark's environment for most adventurers is the silence, if you are completely still and alone your ears will attune to hear your breathing and your heart loudly. A stumble or a yell can be heard for over a mile but the sound is hard to pinpoint because the sound becomes muffled and transforms as it rebounds off the rocky, earthen walls so that a cry for help could turn into horrific shrieks of agony and the growling of a large creature could sound like familiar music.  
Sound plays many tricks on those that are unused to the darkness.
— Narl Irontoes, Dwarf Miner
  Those places that have natural light are dimly lit, either by some stray ray of light that has found its way through reflections into the darkness or due to any of several types of luminous plants in the area. There are a few crystals and metals that can also produce light if the proper conditions have been met, often these are found in Underdark cities providing a dim red, blue or gray light that allows the citizens to see with relative ease. The general rule for light is that the deeper you go, the more rare plant-produced light will become which forces you to rely on finding crystals or mystical metals but lights that move deep in the Underdark usually signify a predator or animal that has the ability of bioluminescence. Whether by natural chemistry or magic there are many creatures that use light as a means to lure others to specific areas, none of these creatures have good intentions. For those that have highly developed Dark Vision, the Underdark is a mysterious world full of beauty and danger that houses incredible rock formation, giant cliffs, waterfalls and views that cannot be found on the surface world but most travelers will never notice such things as they cower in fear from the darkness.

Fauna & Flora

The Underdark is home to incredible amounts of plants and animals that fail to surface in other parts of the world, mostly due to a lack of toleration for the bright sunlight of the world above ground. Many of these plants thrive in environments with low light or absolute darkness. Many of them will die quickly under focused light or develop diseases inexplicably which makes it difficult for scholars from the surface to study them without traveling to their natural habitat. The most prominent types of flora and fauna may seem alien and be completely unknown to those that study botany or zoology as a passion if they have not traveled to the Underdark. There exist few, reliable compendium though two books of some renown in the modern world are What to Eat and What Eats YOU by Percival Yintower and Bestiary of Shadows by Darius Nizz, while the latter has a somewhat informal tone the information is considered to be second to none.   The three main categories of Flora that are of note in the Underdark are called: Luminus Fungus, Lightless Leaves, and Deadly Creepers.   An example of the Luminous Fungus category would be the Blue Lamp, a large fungus that grows in abnormally dry areas of the Underdark that are far from water. Usually, these will grow close to deposits of Iron, Gold or Zap Steel. They resemble common fungus but are several times larger, with some examples being up to four feet in diameter and up to seven feet tall. These mushrooms give off a faint light that is blue in appearance and is concentrated underneath the cap of the mushroom so that creatures seeking the light will often be covered in spores. These spores are not dangerous to most creatures but they can effectively cover a variety of materials to include flesh, fur, cloth, and metal with a shower of spores that will glow faintly for several hours. Many creatures of the Underdark use this plant for food, as the flesh of the mushroom is quite nourishing contains a great deal of water and iron which has led to many claiming it is the adventure's best friend.  
Botanticals by Alfred Cossman
An example of the Lightless Leaves category would be Coffin Clover, a drab plant that is gray in appearance and often appears to be dried up or dead to most observers. Usually, these will grow in damp areas near waterways but they are never found near the openings of caves or near large civilizations as they appear to be sensitive to both traffic and light. Coffin Clover looks exactly like its above-ground cousin, the common green clover but it is considerably thicker and often dry or brittle to the touch. Their remarkable quality is that if anything brighter than indirect torchlight touches their leaves they will immediately reflect that light at several times its original brightness which often blinds nearby creatures for several moments. After emitting the bright flash, Coffin Clover will shrivel and die which leads many to believe that the plant is extremely sensitive to light and upon receiving too much it will overload and expel every bit of energy, magic, water, and light that it has absorbed over its lifetime in a single instance before dying. Due to its tendency to grow near waterfalls, cliffs and other areas that are dangerous to navigate many travelers have been advised to go without light or to procure a hooded lantern as to avoid blinding themselves on unsure footing.  
Seaweed by Helen Shideler
An example of the Deadly Creepers would be the Leather Weed, an aquatic plant that is rare in the darkest of ocean caves but is found abundantly throughout the Underdark. This plant has the general appearance of Seaweed but it is long, unbroken and uncharacteristically black leaves that have a length ranging from 4 to 10 feet in length in most cases with anywhere from 24 to well over a hundred leaves. Usually, these will grow completely submerged near large bodies of water, but occasionally at the bottom of small, slow-moving rivers. Their remarkable quality is that they are carnivorous plants that drown creatures that attempt to drink from the water or to cross in boats with low edges. These plants will silently move with the ebb and flow of gentle water or slow currents and wrap themselves around living creatures, using their heat signature and scent as a means of locating prey before quickly pulling them down to the sandy bottom lakebed. These plants have a spined mouth buried in the sand that leads to a simple stomach and deep root system, these plants are unable to digest large creatures but will instead drown and rend their bodies while absorbing blood tissue slowly over time. The plant receives its name due to its abnormally strong and resilient leaves which are similar to leather in strength and their reaction to being dried which causes them to shrink and toughen even further. Many adventurers have met their end by checking on the sunken body of another traveler only to be pulled down to their own doom.   The three main categories of Fauna that are of note in the Underdark are called: Horrors, Humanoids, and Insectoids.   An example of the Horrors would be the Hook Horrors, large creatures that resemble flightless birds with hook-like appendages made from bone in place of their wings. These creatures will often lurk on cliffs, ceilings, and outcrops where they would easily be missed by predators and remain hidden from creatures that can become easy prey. These creatures will drop from great heights with an uncanny amount of provisions to sink their powerful hooks and beaks into prey while pinning it to the ground so that they can begin feasting immediately. If attacked they will often retreat as they favor quick meals, not bothering to dispatch their catches but rather to start feeding while they are still alive. This has gained them some notoriety among even surface dwellers that have begun to turn their lamps and spells towards the ceilings of large caves so that they do not walk into the danger zone of a cluster of Hook Horrors looking for easy food. When fighting head-on, these creatures are usually manageable for those well versed in monster combat and unless they come in large numbers from above most caravans can fend them off with effort but it is the lone traveler that finds themselves wary of the Hook Horror.   An example of the Humanoids would be Kobolds, small creatures that resemble lithe, short dragons that walk on two legs with a tendency for traps and cowardly tactics. Kobolds and their kin are found throughout the Underdark, they build large settlements that span over a mile of tunnel networks with only a few caverns dedicated to those of importance in their societies. They will only attack when they have an advantage or when they are pushed into a corner and often rely on escape tactics, underhanded fighting, and traps to ensure their safety. It is said that chasing a Kobold into a small tunnel meant only for Kobolds is more foolish than trying to sneak past a Drow sentry. While these are some of the smaller humanoids in the Underdark they highlight the reliance that they have on small tunnels and passageways that hamper the movement of larger creatures, such as adventurers that may want to retrieve magical items that a Kobold has pilfered from them in the dark. Despite being small creatures that are more troublesome than deadly, most Kobolds align under a Dragon that they worship and look to for protection so there is never a guarantee that chasing a Kobold into its lair will not lead you into the presence of their protector.   An example of the Insectoids would be the Black Mantis, large creatures that are identical in every way to the common Praying Mantis but possessing a height between 6 and 8 feet tall. While the idea of a Praying Mantis may not seem threatening to surface dwellers many that frequent the Underdark fear sharp corners that they cannot see around because a Black Mantis can avoid the detection of even creatures that have developed the ability to see in the dark. They are able to keep completely silent and still, disappearing into their environment and bypassing the senses of most creatures as long as they do not move. Then they will wait for a creature of the appropriate size to devour to walk near enough to seize with armor appendages that skewer and hold their prey in place. These voracious eaters will attack anything smaller than themselves and sometimes creatures that are much larger depending on how long they have gone without food. Even when faced head-on these giant insects will fight back with an impressive exoskeleton that puts most armor to shame, many Drow hunt the creature for this very reason and some of them have ensured that the species survives due to their carelessness.
Contested Ownership
Goblins Factions
Orc Factions
Drow Cities
Duergar Kingdom
Dwarven Kingdom

Underdark Provinces


This province is located directly under the Elven Forest and the majority of the Spine of Kallex, it extends north and south to act as the foundation of the Western third of the Northern Continent with some theorizing that it extends up to the Giant Steps Three but this has yet to be proven. The major faction in this area is Drow cities that act as independent city-states. While they see each other as allies when other dangers and hardships are concerned they are very autonomous and are controlled by powerful family lineages.   The other opposing faction of note in this area is the Mountain Orcs who live as nomadic, warfaring tribes that often attack trade routes and will stray up as far as the Great Tunnel in search of battle. While they do not control specific areas they are considered to be a powerful faction that can show up at any given point in this province whether you are close to the surface or several weeks worth of travel from it. They are the same faction of Orcs that can be found above ground living throughout the Spine of Kallex, known for their aggression and long-standing enmity with the Elven Kingdom.  


This province is located under portions of the Fairy King's Wood and the Great Plains, extending north under the Neither Tundra and south to some of the islands of the Zóni. This is considered to be the largest province of the Underdark, characterized by an abnormal amount of Horrors and Large Insects. The major faction in this area is the Drow cities that act as independent city-states. These states see themselves less as allies due to larger amounts of geographical separation than the other provinces due to drastic drops in elevation and waterways stemming from the presence of Lake Idryl. These cities are autonomous and are controlled by powerful family lineages.   The other opposing faction of note in this area is the Endless Green, a super-tribe of Goblinoids that are spread throughout hundreds and thousands of caverns. These Goblinoids are known for using the ever-changing passages that connect the Underdark to the world above and they will rarely use known or permanent paths. Because of this, they are constantly scouting and searching for new ways to move about their province so that they may steal, raid and enslave small settlements and travelers. They are exceptionally well known for their darker trade in the Underdark and generally have information on most of the happenings throughout their province and others.  


This province is located under the Timbers of the Furnace, extending south under the Little Greenwood and north under the Neither Tundra with several outlets into the open sea through the use of subterranean lakes and rivers. This is considered to be the smallest province of the Underdark, though this may partially have to do with the expansive ground that has been reshaped and held for thousands of years as the land of the Dwarven Kingdom. The Dwarven Kingdom is such a different place, that it is often not considered part of the Underdark but the Duergar Kingdom is much farther underground and properly considered to be one of the major factions of the area. This Kingdom is a series of fortresses and towns that are connected by rail that navigates dangerous gem and metal mines.   The other opposing faction of note in this area is are the tribes of Trolls, though not united and having no official name they are quite populous and highly territorial. They fight with each other just as often as they fight with travelers, merchants, and established areas of the Duergar Kingdom which leads some to believe that the presence of Trolls caused the Duergar to develop their natural use of size-contorting magic so that they could more easily repel these creatures. These trolls are of a variety that detests sunlight above all else and will rarely be found close in proximity to the surface, it is said that if they are exposed to the full strength of the noon sun that they would turn to stone.  


This province is located under the western edge of the Crumbling Wall, extending south to the Chromatic Peaks and encompassing the Lake of the Five Paths. This is considered to be the province with the most water, lakes, and rivers abound in this area with flooded caverns that often separate or completely hide civilizations away. You can also find aquatic races that commonly live out in the seas in this area of the Underdark, most of them have migrated to these secluded caverns thousands of years ago by way of passages long forgotten. There is no major faction in this area, though species of note that have presence are Drow, Duergar, Lizardfolk, Orcs, Kobolds, Goblins, and a variety of aquatic dwelling races that usually have peaceful lives due to natural barriers from the other races.   This area is also the least traveled and charted of the world with more than a few Dragons taking up residence as the Southern Continent struggles to completely house the many species of Chromatic Dragons that exist within the world. The southern portions of this province are considered strange because the frozen temperatures of the surface world reach down and permeate them despite logic dictating that the world should warm the subterranean tunnels. Scholars have yet to uncover this mystery due to a lack of willingness to explore the dangerous area.  

Alithinós Nótos

This province is located under the vast Forest of Mytos, extending north to the Scorched Plains and south past the Oracle's Seat. This is considered to be the province with the least amount of settlements as it is mainly controlled by monster races that live in the Scorched Plains and throughout the vast rainforests that cover most of the Southern Continent. The major faction in this area is called the Rotting Gnolls and is the only place in the Underdark where undead are found in abundance, leading to the strange amalgamation of more common Underdark flora and fauna that have been corrupted and twisted by dark magic. Gnolls form packs, sometimes combining into super-packs when they have an enemy or a settlement that they wish to overcome and have been responsible for the lack of built-up areas in this section of the world.   This area is known to have been the province where many demons and dangerous creatures have been sealed away in ancient times. It is said that at one point it was the most populated area of the Underdark and controlled by the Drow almost exclusively but a catastrophe occurred in distance history that left the area desolated and in control of monster factions. The Drow have not shown interest in trying to purge or reclaim this area in recent history due to the impacts of the Eldritch War on the entire world.  


This province is located under the largest Deadzone in the world, it extends north to the Sunset Fields and south to the Island of Blessings to encompass the area that was once said to be the country of Djinn. This is considered to be one of the stranger provinces in the Underdark because most humanoid creatures avoid it, this area of the world is desolate and largely devoid of most intelligent creatures. A traveler can go an entire month without seeing another creature that can speak much less reason, but they will see many of the darker creatures that the Underdark has to offer to include creatures that do not originate from Isekai. Things that follow rules of magic, power, and physics seeming to defy psychological understanding but there is much debate about where these creatures have come from and why they were drawn to this area.   This area is known to have highly aggressive Drow cities that are distrusting of even their own race, the same can be said for any Kobold factions that can be found here. While Kobolds as a general rule are known to be cowardly these factions are usually terrified of encountering others and have an overabundance of traps in their areas, some that have appropriated an uncommon amount of magic for creatures of their skill. This is often considered the most dangerous area of the Underdark to stay in for any length of time as the caverns and paths change more often and more sporadically, no one is sure what fuels these intense and swift changes in geography.  

Other Provinces

Many smaller provinces exist throughout the Underdark with their own quirks and unique characteristics, these are often attached to islands and some of them are only accessible from passages along the seafloor. While it is often thought that every province of the Underdark is connected if you travel deep enough these smaller provinces seem to perpetuate that idea as they have no lack of depth despite not having as expansive or sprawling areas to exist beneath.

Blue Lamps

Blue Lamps have a plethora of uses to Underdark dwellers but the stem of the plant has a peculiar filling that is similar to the consistency of jelly or thick mucus. This slime has high nutrient qualities that make it useful for medicines, crop fertilization and it is highly visible if soft lights are shined upon it. Many secret messages and survival markers have been made using this common blue slime. People have left messages, crude arrows and even drawings along stony walls or trails in the Underdark to warn or sometimes trick other travelers that come behind them. If dim torchlight illuminates the slime it will react incredibly well and glow with a sparkling, blue quality for about 10 seconds before returning to normal.   The slime must be left alone, for at least a full minute in order to repeat the process and cause it to illuminate itself again. This ability of the slime shows that the application is recent because it only retains the quality of illumination for 7 days after it has been separated from the fungus that houses it.

Coffin Clover

Coffin Clover has the strange name it does not only because it dies when exposed to light but it is said to have varying degrees of interaction with creatures that have lived and died. All creatures that have been petrified or turned to stone will have Coffin Clover that accumulates on them within the course of 48 hours, no one is sure why this occurs or what the correlation between petrified creatures and the plant is. The Duergar also have folk tales buried deep in their history that speak of exposing the plant to different types of light or magic and being greeted with different results. Some speak of misfortune, others of blessings but it is said that the name of the clover originated in ancient times when the Huntress chased those creatures that had offended her deep into the Underdark.   It is said because she transitioned into a god aligned with death that these plants reacted to her presence and are forever forced to dwell in darkness and die upon the presence of light. However, fully grown Coffin Clover are exposed to the scream of a banshee that has fulfilled its vengeance then they will transform into a fine dust that can revive the dead and return minds that have long been lost to insanity.

Hook Horror Kits

While Hook Horrors are often seen as creatures to be afraid of, they can also be used as a natural resource to create many useful items. The hooks that replace any sort of wings on these strange birds are made of bone and quite sturdy to the point that they can be fashioned into short swords or climbing implements that have the added benefit of being immune to rust. Their talons are wickedly sharp and several times larger than a standard eagle's, which has led many to carve them into caltrops or to fashion as the ends of whips or for gloves that monks and brawlers might use. These are especially popular among Drow Rangers who are notorious for emergency and back-up weapons that can rip flesh or cause excruciating wounds to allow them to escape from deadly creatures.

Black Chitin

The Black Mantis is among the most dangerous of stealth-based predators in the upper tunnels of the Underdark, but their exoskeleton is known for its durable quality and a sought after material in making armor. It is lightweight, highly resistant to bladed weapons does not burn easily. These factors, and its natural camouflage properties that persist after the creature's death make it an incredible material for armor and shields. The armor is just as strong as metal if it has been treated properly and lacks the disadvantage of being as noisy as metal scales or rings.   The secondary use of this creature's exoskeleton has been repurposing the spiked arms for use in traps and defensive structures. Many a Duergar trap has been triggered by trespassers that step on a pressure plate only to have a dozen spiked mantis arms spring from a slot in the wall, seizing and pinning whatever happens to be standing there at the time. The spikes on these arms are long, curved, and grown in a way that leaves wounds that are difficult to heal if one simply rips their flesh away from the restraint.

Cover image: Isekai Cover by Landscape via Artbreeder, created by RiverFang


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