The Darklands

The Darklands are an immense series of caverns, vaults, and passages under the surface of Golarion. While entrances to the Darklands can be found throughout the world, it is not common for travellers to venture into the almost universally unsafe depths, making the subterranean realm almost a world unto itself.

Geography
The Darklands can be divided into three distinct layers, each with their own characteristics, inhabitants and hazards.

Nar-Voth
Nar-Voth is the uppermost of the three regions—the "wilderness" of the Darklands. Most explorers of the Darklands are only familiar with this region. It is mostly composed of a series of cave networks connected by lengthy twisting tunnels. For a cave network to be considered part of this region, there must be a connection to the region of Sekamina below.

Sekamina
Below Nar-Voth lies the largest realm—Sekamina. This region consists of a vast underground warren of tunnels that span continents. Found here are also great underground rivers and lakes. Most of the entrances to Sekamina require traveling through Nar-Voth first, but it is believed there are exceptions.

Orv
The deepest realm of the Darklands is the mysterious and dangerous layer of Orv. Here, many immense caverns known as vaults host a wide array of underground oceans, caverns with jungles and artificial suns, entire nations of warmongering humanoids, and regions containing mountain ranges and vast deserts. Typically the denizens of the upper Darklands think of the vaults with the same sense of fear and wonder that surface folk do for them, thus only the absolute bravest of adventurers ever seeks out these deep subterranean worlds.

Entrances to the Darklands
Just because a cavern or dungeon is deep does not mean that it is part of the Darklands. Most such places are simply natural or artificial caves that connect only to the surface and do not delve any deeper. In order to truly qualify, an underground location must possess connections to the greater system of tunnels and caverns of Nar-Voth or Sekamina. These entrances range from the once great Dwarven Sky Citadels to the infamous Pit of Gormuz and aren't always as obvious as one might suspect.

Darklands of Tian Xia
The Darklands below the continent of Tian Xia is in some ways similar, but in other ways very different from that of the Inner Sea region. Like its western counterpart, these Darklands are broken down into three levels: Nar-Voth, Sekamina, and Orv. What is most different, are their inhabitants; the Darklands of Tian Xia have no major populations of duergar, drow, or derro. Instead, they are home to haunted clockworks, denizens of Leng, oni, and Cave Giants.

Travel
Finding one's way through the Darklands can be a dangerous and time-consuming undertaking, not only because of its belligerent inhabitants, but because the physical structure of its tunnels and chambers complicate travel. Although more linear, artificial tunnels and grottoes exist (most notably in Orv), the large majority of passages in the Darklands are the result of natural processes and are twisting and confusing, often ending in cave-ins, pits, or other impassable obstructions.

History
The Darklands are the most ancient region of Golarion, but much of it remains a vast wilderness with a few scattered nations and city states. Many of the tunnels, especially in the region of Orv, are believed to have been originally created countless years ago by the mysterious Vault Keepers. The fact that these tunnels and caverns remain intact today speaks volumes about the power of these enigmatic entities.

During the Age of Legend, the Serpentfolk ruled large parts of the Darklands. They were defeated by the Azlant, and much of their empire was reduced to ruins. Following Earthfall, many Elves fled into the Darklands—and later on in the Age of Darkness they were transformed into Drow.

Later, the Dwarves began their Quest for Sky, abandoning their homes in the Darklands for the surface world, and driving their enemies the orcs up to the surface before them.

Inhabitants
The Darklands play host to as wide an array of creatures as the surface world. Some are intelligent and civilized, while others are bestial, savage, and primal. Throughout the three layers that make up the subterranean domain, one can find any number of settlements which rival their above-ground counterparts. Most of these are in the upper layers of Nar-Voth and Sekamina, but even some of the most deadly vaults of Orv are teeming with life.

The 'civilized' races that inhabit Nar-Voth include Duergar, Mongrelmen, Troglodytes, and Derro. Most of these races are relatively isolated, tribal, or small, with the exception of the Duergar, who occupy a number of abandoned pre-existing Dwarf cities.

In Sekamina are found Grimlocks, Ghouls, Vegepygmies, Aboleth-thrall Skum, Morlocks, Serpentfolk, Svirfneblin, and Drow. Despite the wide variety of humanoid races living here, the Drow tend to be in control of the largest portion of Sekamina.

Inhabiting Orv are Neothelids, Urdefhan, intellect Devourers, Gugs, Gore Weavers, and Umbral Dragons. Most of these creatures are isolated to specific vaults, although the Urdefhans can be found in many places.

Darklands hazards
The Darklands are a dangerous place, a fact to which anyone who has ever journeyed there can easily attest. This is partially due to the fact that compared with the surface world, food sources are extremely scarce there. This makes the competition for those sources that do exist that much more intense. Perhaps because of this, the creatures and inhabitants of the Darklands, all up and down the food chain, are quite deadly. In addition to the obvious threats presented by deadly predators, insatiable scavengers, and diabolical ambushers, there are the more mundane, but nevertheless lethal, hazards of the Darklands.

Air
Most of the air found below is safe to breathe, if a little stale. In areas blocked off by tunnel collapses or water, or near sources of noxious gases, however, it can become dangerous to travellers. The air near large magma flows can be quite caustic, and poisonous gases like carbauxine or quickdeath can lead to an even quicker demise.

Fungi, slimes, and molds
Most plants have little chance of surviving in the dry, dark caverns of the Darklands. There is simply not enough water or light (with the exception of some of the stranger Orvian Vaults) for them to live. Instead, those who live here largely rely on the various fungi, molds, and slimes for sustenance. But like everything else in this hostile environment, these plants can be dangerous as well. Among the fungi, these include the rot-inducing violet fungus, the animating slaver fungus, the brain-damaging cytillesh, and twilight mushrooms. The dangerous molds include the cold-exuding brown mold, russet mold which gives birth to vegepygmies, poisonous yellow mold, and life-draining ghost mold. Harmful slimes of note include the corrosive green slime and the innocuous-seeming olive slime.

Creatures
The Darklands are home to many strange creatures, including the stirge hound, ether frog and ghost bat.