Underdark

The Underdark is a network of subterranean tunnels, caverns, seas, and rivers that spans the entire world.
Lightless but far from lifeless, the Underdark is home to a dizzying array of creatures, from the civilized but
unforgiving drow to carnivorous monsters that haunt the darkness.

## The Upperdark:

Comprising approximately the first 3 miles of depth, the Upperdark is a realm shared by drow, duergar
(gray dwarves) and other dwarves, and deep gnomes. Here the races of the World Above (as Underdark
dwellers call the surface lands) intermingle with those who dwell in darkness. Encounters here range
from drow or dwarf patrols to all manner of wandering monsters and humanoid hunting parties. The
largest concentration of plaguecaves (see below) are encountered in the Upperdark.

## The Middledark

The larger drow and deep gnome cities are located in this layer, 3 to 10 miles below the surface. This
area was relatively insulated from the effects of the Spellplague, so many of the drow cities remained
unchanged and plaguecaves are rare here. However, the concentration of faerzress (see below) increases
dramatically in the Middledark.

## The Lowerdark

Below 10 miles lie the most dangerous and alien reaches of the Underdark—places that even drow
seldom venture into. There are more dead ends and isolated vaults in the Lowerdark, and the faerzress
here is more powerful, covering larger and larger areas with more arcane effects the deeper one
descends. The walls between planes can be thin in the Lower dark, with tunnels reaching into the Nine
Hells, the Abyss, the Shadowfell, and other domains of chaos and evil.

## Settlements and Features

The Underdark is a whole world of its own, with dangers magical and mundane, and a varied ecology that
supports millions of sentient beings and ravenous monsters.

## Faerzress:

This magical radiation, a remnant of a work of dark high magic, is scattered at random throughout the
Underdark. Faerzress interferes with spells and the effects of magic items in often unpredictable ways,
but has the most detrimental effect on divinations, teleportation, and most other forms of magical travel.
Faerzress can take a variety of forms, from clusters of glowing rocks to a barely discernible change in
temperature. Natives of the Underdark know to look for colonies of magivores, a form of life somewhere
between a plant and a coral that feeds off the magical radiation. Magivore colonies and their airborne seed
spores provide a ready source of food for the lower forms of animal life in the Underdark.
Particularly talented wizards are capable of rituals that can alter the form of a pocket of faerzress in
specific ways, but those rituals tend to attract the attention of the creatures that feed on magivores—and
the creatures that in turn feed on them.

## Earth Nodes:

Streams of invisible power run beneath the earth, occasionally crossing and collecting in a single spot
like river flowing into a lake. Such rare collection points are called earth nodes.
Earth nodes are valuable sources of raw magic, collected from the deepest core of Toril, which flow into
a central point indistinguishable from the surrounding territory. They are difficult to find but enormously
valuable. Whole Underdark cities, and no small number of jealously guarded citadels, have been
built around earth nodes to harness their power. See “Earth Nodes” on page 51 for more about these magical
phenomena.
Earth nodes sometimes generate random effects, such as creating localized earthquakes or granting
a stoneskin effect to creatures nearby. In the last several decades, numerous efforts by drow arcanists
have resulted in a network of earth node portals that ignore the effects of faerzress.

## Sensing a Node:

An earth node’s power isn’t visible to the naked eye, but such points are often eerily beautiful or bizarre.
Anyone who approaches within 30 feet of the edge of an earth node’s power is entitled to a DC 20 passive
Arcana check. (The check can also be made by a creature who is searching for the phenomenon.)
Success reveals the presence of the node to that individual. Anyone who senses the node’s power can use
that sense to map its boundaries, but doing so takes time, requiring physical travel and observation.

## Plaguecaves:

The Spellplague passed through the Upperdark, affecting the creatures, and terrain there in many of
the same ways it altered the World Above. Plaguecaves are areas where active pockets of Spellplague
were trapped in enclosed spaces, over time twisting the natural structure of caves and tunnels into
bizarre domains. Here streams flow through the air, water falls upward, and earthmotes float about like
fish in a bowl. Some Underdark residents embrace plaguecaves, especially the spellscarred illithids known as the
Mindplagued. Most Underdark residents stay away from plaguecaves—drow in particular avoid them at
all cost.

## Abysses:

Surely the most dangerous of the Underdark’s natural formations, these deep pits, holes, and chasms don’t
just appear bottomless. Some empty into other planes of existence, including realms such as the Abyss itself
and the Nine Hells, out of which crawl many twisted creatures that inhabit the Underdark.

## Seas, Lakes, and Rivers:

Water is plentiful in the Underdark and takes as many forms belowground as it does above. Streams of
fresh water meander through tunnels, collecting into pools or joining with other streams to form rivers that
empty into lakes or seas to rival some of the larger inland seas of the surface. Some caverns are entirely
submerged, home to aquatic monsters and humanoids, and waterbound aboleths.

## Climate:

Though most of the Underdark stays at a constant, cool temperature, with air circulating through shafts
and tunnels, parts of it can experience something like weather. Cold areas and hot spots, wind tunnels, and
strange, billowing mists appear at random intervals.