The Deep Lands

There is a vast subterranean realm deep below the surface of Aestas, known by many names; most of these have the same meaning: "The Deep Lands". They are a region of fear and mystery for the surface world, as the inhabitants are strange and terrifying, some twisted and evil, others ancient beyond understanding. Located between one to five miles below the surface, the Deep Lands are any of several immense cavern networks. These major networks are not believed to be connected to each other, as they are located on several different continents. Natural caverns are not uncommon on Aestas, but only a few go as deep or stretch as far as the Deep Lands. The largest caverns that are part of the Deep Lands are far too large to have formed naturally. Nobody knows their origins, though sages have pondered it for centuries. 

One of the most bizarre facets of the Deep Lands is the ambient magical energies which suffuse all parts. As there is no sunlight to be found, all plant life has adapted to instead live upon this energy. Most life forms in the Deep Lands are similarly adapted to use this energy source. The downside to this magical energy is that it makes spellcasting unpredictable. Any time a spell is cast, roll a d20. On a "1", roll on the Wild Magic table (PHB p.104) and apply the results. Some areas of the Deep Lands are magically active, and the odds of such a magical surge are considerably higher. 

Regardless of the specific network, the geologies are all fairly similar. Enormous caverns miles across stretch beneath mountain ranges and hill areas, and are connected by endless warrens of smaller tunnels.Typical major caves are 1d4-1 miles wide, and 1d6 miles long, with connecting tunnels and 1d4-1 settlements of various sizes. many have waterways of some sort, especially small pools and ponds, though a few of the deepest ones have actual lakes of considerable depth. Major caves are connected by warrens of tunnels, either natural ones (if a watercourse), or tunnels dug by ravenous Purple Worms. Well known regions of the Deep Lands include the "Far Below", of the Keller Lands (a corruption of the Dwarven name for the Deep Lands), and the "Gezani Reaches" beneath the "Plain of Soot and Ash" in the Uthman Lands. Both stretch for thousands of miles, and contain whole kingdoms unknown to the surface world except in legend.  In very broad terms, the Deep Lands are categorized as either "Settled" or "Wilderness".

Settled Areas - These areas are marked by roadways or at least paths through the fungal forests, patrols by the residents or their minions, and structures that are built and manned to control access. Many of the races which inhabit the Deep Lands are actually members of surface dwelling races that were forced underground and adapted over millennia to life underground. Duergar are the dwarves who moved deep below ground in millennia past. They are the dominant power in many upper regions, living in small free standing towns with forts built into the walls; populations smaller than equivalent Dwarven cities, with slaving economies. At the same level but in smaller chambers live the Svirfneblin, who reside in towns built into the walls of their chambers. Deeper down in the earth, Drow settlements can be found, each one with a well-guarded gateway to the Shadowfell. Deepest of all live the mind flayers, and their kuo-toa and grimlocks slaves and worshippers. 

Wilderness - Whether vast fungal forests or barren side caverns, these are regions unclaimed by civilized beings. They are home to Troglodytes, which live in upper parts of the Deep Lands, Quaggoths, which dwell in the chasms and rifts of the region, and the occasional Myconid circle. They are joined by all manner of beasts. Animals are large and pale, feeding on fungi if herbivorous, or anything at all of carnivorous. Giant Spiders, Carrion Crawlers, Gricks, Deep Ankhegs, and Hook Horrors are the most common. Aerial predators include Cloakers and Giant Bats. Otyughs are a bit like bears in the Deep Lands. Lakes are inhabited by blind, pale arthropods, some of great size. In addition to the various beasts are "hermits", creatures powerful enough to be left alone. Hags, Ogre Magi, and the like often have small homes in the middle of forests, alternately preying upon or doing business with local settlements. The occasional large lake will be home to an aboleth, usually served by Chuul minions and thralls.