Labirynth is probably one of the most insane things ever created by a sapient specie inhabiting Gates of Eternity. It is an effect of several thousands of years of hard works of many generations of karathians. Their specie had its golden age during the Archaic Period of Preimperial Era, when entire surface of the planet was united under boundless might of Ancient Dragons ruled by The Eldest. The only specie that refused to become their subjects and worshippers were the Karathi. Instead, they decided to flee to the only place where the Dragons couldn't follow them. They fled underground. Of course, they couldn't be satisfied by simply digging out small holes in the ground and then living there, in permanent fear, growing magically modified crops. What sort of life would that be? Instead, they decided to create something great. They build Labirynth.
Labirynth is a massive cave system that is actually better described as underworld. Much bigger and more expansive than surface. It starts at circa 150 meters beneath and it ends at depth of circa 15 kilometres. It is divided into more or less ten consecutive layers, each of them more and more dangerous... and hiding more and more wealth to excavate. The only thing that actually holds it working is a truly astounding magic, made to last eternally. Temperature in Labirynth never rises above 30C (in most cases it is from 5 to 25C), while it should reach MUCH more inhospitable level, especially at lowest tunnels. Air is permanently filtered by magic anchored at certain segments of tunnel walls, which makes it breathable even fifteen kilometres beneath the surface. There is even magical lighting, which ensures that most of it is covered by shadows, but isn't pitch-black. Some areas are even lit to almost surface day-level via magical lighting but an airborne aether consuming genemodified mosses installed into lamp-like constructions. There are of course many segments where that magic died, probably under some extreme circumstances. But they are few and far in between. In most cases being places with much increased survival difficulty... but also hiding undiscovered wealths. Labirynth is also very resistant for cave-ins and earthquakes.
Depending on the region, Labirynth has from one to ten layers, that are interconnected in a majority of ways. Elevators in ancient karathi ruins or connector tunnels going up or down. There are also tunnels, elevators, ladders etc. made by civilizations that control this place today. This layers - and Labirynth as a whole - can be divided into four main parts. Oversee, known also as Nearsurface, Shallows, and by many other local names. It includes layers I to IV. IT is mostly inhabited by civilizations that are heavily connected to the surface, in most cases being a vassal states of Imperium of Karadia. Most of it are classical caves (sometimes so big that there are clouds forming beneath the ceiling). This place is filled with abandoned karathis towns and villages (often settled by newcomers), connected with each other with the so called Old Roads, a well maintained karathi roads with good lighting and even floor. It also includes the so called Zero Layer, which are additions to the Labirynth made by its new inhabitants - for example dwarven/dark elven fortresses in the mountains, connected by the tunnels they digged with the First Layer. This place isn't particularly dangerous. Air can sometimes be a problem (the farther from karathi buildings, the worse it gets), and local fauna (and sometimes flora) can be problematic. There are also sapient species, which are sometimes the worst thing one can encounter. There is also a risk of getting lost and dying of hunger or dehydration. There are still secrets to be found, but probably nothing really extreme. Undersee, also known as Lower See, Blackwater, Karathian Sea, and Sea Beneath. It includes Layer V... and is probably the most shocking and insane part of Labirynth. A massive hollow... filled with water. Lots of it. Karadia-size sea, hidden several kilometres below ground, with many rock pillars (sometimes many kilometres wide) that house outpost of labirynthian civilizations. With ships navigating the perpetually dark waters of the Sea Beneath, being the fastest way to travel through Labirynth. If not for the karathi magic reinforcing the rocks, the entire continent would probably collapse on itself... It has, of course, its own fauna. Mostly aquatic though. There are also pillars that noone likes to come too close to and many other secretive places hidden beyond the light of navigational buoys that mark the sea routes. It is also an unofficial borderline between the Nearsurface and the Depths. Depths, known also as Bottoms or the Deep Labirynth. It includes Layers VI to IX. It is a place of... overwhelming strangeness and multitude of dangers. There are some civilizations living there, most of them either very xenophobic and isolationist... or under direct control over forces of Pentagram from the nearby Abyss. Twisted into pain-worshippers, or something even worse. Local fauna is insanely dangerous and sometimes borderline cruel, twisted by evil seeping from the lands even lower. Thousands of books could be written about things that moved in to the cities and fortresses the karathi abandoned. Secrets and treasures are everywhere... and so are dangers. Abyss or Edge, is the so called Layer X (though they can be something beneath it). It is one of chief domains of Pentagram in Real World, traditionally attributed to Discord. It is believed that the secret to dissapearance of karathi is hidden there... but there is no way of knowing, since only the strongest beings (Abyssal Knights, Chosen Ones etc.) ever went there and returned to tell the tale. Abyss was created when the unknown lands deep beneath the surface by the collective fear of what might have lurk there, with both surface and Labirynth civilization contributing equally. Rather than organizing a coherent assault, the forces of Pentagram here are permanently in a state of infighting, Aberrants fighting against each other with only slightly lesser fervent that against mortals.
Fauna & Flora
Too countless to describe. There are entire ecosystems there, living in aether-rich environment that changes most of local liveforms into beasts. Some of these ecosystems grew in environment absolutely alien to what lives beneath the sun. Spelunkers tell tales of mushroom forests, caves filled with acidic lakes, places that seem to exist in additional dimensions... There are also entire places classified as magical anomalies. Breaches into alternate realities and another worlds. Caves that are alive. Places that bleed. Areas where crystals breathe. Things that deny description. The deeper one descents, the more insane his surroundings become. There are titans - a variety of powerful non-daemonic creatures, each of them unique, that roam the abandoned tunnels, devouring everything they encounter. They are Labirynthian Gods, according to some ancient gods of karathi long degenerated into merely powerful deities without equal on the surface. Thousand another things, that one has to see for himself to actually believe in.
Endless number of stone, many types of crystals, quite strange wood, veins of pretty much every type of ore known to mortals... and that's without mentioning more crazy and unique things...