The Canker

Delve too deep and The Canker will swallow you whole.

- Dwarven Saying

The Canker is one of many names given to the corrupted tunnels and caverns that run beneath the surface.
Not all sub-surface terrain is part of The Canker. Rather than being defined by a particular depth or biosphere, as would be academically preferable, the reach of the Canker is described by those who brave its darkness as an overwhelming sensation; dread, foreboding, and repulsion to the ground they walk upon.
The proximity of the Canker to the surface varies wildly, both across time and location. Typically it is thought to lie hundreds of metres deep, but historical accounts have described The Canker spreading across the surface. These records are unverified but align with perilous moments in history, and it seems impudent to discard the notion as a fanciful one.

Geography

The Canker is dominated by twisting networks of tunnels. These can stretch on for miles before reaching abrupt and featureless endings, or create looping and nested complexes that can overcome even the greatest of Dwarven Vanguards. The passageways range wildly in size, from inches wide to as much as twenty yards wall to wall. 
Throughout these tunnel systems live many of the "simple" creatures of The Canker - some carving out a lair in nooks and crannies, others hunting for easy prey in The Endless Night. Many accounts from adventurers who have dared The Canker have described walking for days on end without encoutering - or at any rate, noticing - other life forms.
Scattered throughout the Canker are open spaces, miles or more across. These house the sociocultures of The Canker, including those recognisable Peoples such as Drow and Duergar, along with more alien examples such as Illithid and Kuo-Toa, and Nothing-touched abberations such as Beholders.

Ecosystem

Lacking the life-giving warmth that blesses the surface world, The Canker is thinly furnished with flora. Fungi and lichen provide the basis of the unsteady food chains that exist here - although naturally, they are well equipped with defence mechanisms and have developed greater degrees of mobility, autonomy, and even sapience.
The food chains of The Canker are long, leading to great accumulation of the foul substances and magics of that place into its greatest predators. Given the problematic nature of Spontaneous Apparition, it is rare for an enclosed ecosystem to develop and remain stable.

Localized Phenomena

Cankerous Vagrancy

Though made of stone, the tunnels of the Canker vary far faster than petrological timescales, especially nearer to the surface world. Fissures connecting benign cave systems to The Canker have been reported to open and close within a year, though many "stable" connection sites are documented having lasted for centuries.
This makes protecting surface civilisation against The Canker an endless task - while infrastructure can be built nearby wherever such a fissure appears, it is impossible to truly keep track of the threat. Efforts are therefore concentrated to the more stable sites, when they are discovered, and monitoring for activity of agents of The Canker.

Spontaneous Apparition

The Canker is riddled with the sudden appearances of creatures in a physically enclosed or isolated spaces. Any stretch of tunnel, crevice or cavern left undisturbed holds the potential for Spontaneous Apparition. Various experiments have confirmed the phenomenon - although far more have ended in tragedy, and the rates and nature of these apparitions have not been mapped in any meaningful detail.
While Spontaneous Apparition might appear to be the result of teleportation magic, most denizens of The Canker are individually incapable of such feats of magic. The phenomenon is widely agreed to be a latent effect of either the toxin that afflicted U'Dur during the First War. However, the frightening possibility remains that these apparitions are concerted effort from some unknown enemy.

Natural Resources

Having infested deep in the Bones of the World, the Canker is enriched with all the natural resources one would expect to find in a typical sub-surface setting - precious metals, gemstones and useful ores.

History

The Canker can be traced back to First War. In the depths of the fighting, Hiqra's uneasy alliance entertained all possibilities in pursuit of victory - even the destruction of the world itself. Hiqra tempted Apdos to fell cause by offering ultimate power - if all of Durath were to die, then would not the God of Death reign supreme?
A tremendous weapon was levelled against U'Dur - a spear of shadows, tipped with raw Nothingness from the ragged edges of reality, which Skein had refined into a toxin. When U'Dur was struck by the spear, the toxin was spread into the bones of the world, and all began to crumble and rot.
An alliance of Gods was able to intervene, saving the existence of U'Dur. The damage done by Nothingness could not be reversed, only contained. And so, the wound was pushed to the depths of the world, in order that the mortal denizens might live in safety.
It is unknown what would have happened to Durath had the foul substance not been contained. Perhaps it would have become twisted forever to darkness. Perhaps all life upon it would have been corrupted, falling to undeath. Perhaps a terrible nothingness would have befallen every corner of the world.

Alternative Name(s)

The Underdark, The Night Below, The Endless Night

Type

Underground / Subterranean