Description

The Underdark is an immense subterranean realm stretching beneath the surface of Faerûn and much of Toril. It is not a separate plane of existence, but rather a vast underground world composed of caverns, tunnels, underground seas, ruined cities, and ancient passages extending for thousands of miles.
The Underdark is infamous for its dangers, hostile civilizations, and alien environments. Entire kingdoms exist in perpetual darkness far below the surface, illuminated only by glowing fungi, magical crystals, and strange subterranean phenomena.

Overview

The Underdark is divided into countless isolated regions connected by winding tunnels and cavern networks. Travel is perilous due to monsters, cave-ins, hostile factions, and the ever-present risk of becoming lost.
Civilizations hidden within the Underdark range from powerful drow cities to ancient illithid colonies and forgotten ruins predating modern history.
The deeper one travels, the stranger and more dangerous the Underdark becomes.

Physical Properties

The Underdark consists of:

• Massive caverns
• Underground rivers and seas
• Vertical chasms
• Volcanic regions
• Ancient ruins
• Endless tunnel systems

Natural light is almost nonexistent. Instead, illumination comes from:

• Bioluminescent fungi
• Magical crystals
• Lava flows
• Faerzress radiation

Faerzress, a magical energy permeating parts of the Underdark, interferes with divination, teleportation, and planar travel.

Planar Adjacency

The Underdark possesses unusual connections to multiple planes and cosmic forces.
Certain deep regions show influence from:

• The Feywild
• The Shadowfell
• The Far Realm
• Elemental planes

Some ancient tunnels and magical breaches lead directly into extraplanar realms, especially in areas corrupted by aberrations or ancient magic.

Magic on the Plane

Magic within the Underdark is heavily influenced by faerzress and the environment itself.
Common magical phenomena include:

• Wild magical surges
• Distorted teleportation
• Permanent magical darkness
• Psionic resonance

Arcane and psionic magic are both widespread among Underdark civilizations.

Inhabitants

The Underdark is home to numerous dangerous peoples and creatures, including:

• Drow
• Duergar
• Deep gnomes (svirfneblin)
• Mind flayers
• Beholders
• Aboleths
• Myconids
• Driders

Countless monsters, aberrations, and predators also dwell within its depths.

Cosmological Role

The Underdark serves as both a hidden world beneath civilization and a gateway to stranger cosmic forces.
It is associated with:

• Ancient lost civilizations
• Forbidden knowledge
• Aberrant corruption
• Survival against overwhelming darkness

Many surface dwellers view the Underdark with fear and superstition.

Variations by World

Most worlds possess some form of deep subterranean realm, though their structure varies greatly.
In Faerûn, the Underdark is especially extensive and politically complex, containing major drow empires and illithid settlements.
Other worlds may possess smaller cave networks or entirely different underground civilizations.

Planar Travel

Travel into the Underdark usually occurs through:

• Deep cave systems
• Ancient ruins
• Mines
• Magical portals
• Chasms and sinkholes

Navigating the Underdark without guides or maps is extremely dangerous, and many travellers never return.

Locations