Hollow World AKA Underdark

Deep within the heart of Thylea is the Underdark, also known as the Hollow World. It is a vast labyrinth of winding caves and tunnels with the occasional large opening. The bleak demeanor of the Hollow World gives the residents there an overarching feeling of dread, dispair, or some-times insanity. Planar scholars argue that this is due to the way it was created. Some of these scholars say that the Underdark is the inside of Thylea, which was empty except for the pain and suffering that she felt. Others say that it is the remnants of the land that existed before Thylea that her body lied upon at the time of her death.
The area is home to a wide variety of creatures, most of them wanting to eat each other. In addition to the obvious inhabitants, like worms and spiders, few sentient creatures make their home there. Troglodytes constantly war with each other over trivial matters, but occasionally band together to fight their hated enemy, the driders.
The driders are said to be the children or descendants of Sotera, a nymph who wished to be the right hand of Arasta but was driven insane and transformed after a brief encoun-ter with all of eternity. The driders now weave their webs to make impassible tunnels and traps for the troglodytes, which they use as sacrifices for their various rituals to Arasta.
Serving as neutral parties in the troglodyte-drider war are two cities: the Majestic Kingdom and the Nekropolis of Asphodel. The Majestic Kingdom was once a safe haven for refugees of the First War that fell after a citizen discovered a divine artifact, leading the kingdom to be infiltrated. Asphodel is a city of ghosts, gathered here by Charon the Ferryman for his own nefarious plot.

Geography

LAY OF THE LAND The Underdark is a large network of honeycombed tunnels with the occasional cave, chasm, lake, or a large opening. Such large openings tend to serve some other purpose than just being an empty pocket in the ground. These can include dens of various beasts and monsters, lairs for more intel-ligent creatures, and larger phenomena of the Hollow World.
While many of the tunnels and caves are natural, caused by tectonic plates and the occasional earthquake, many are made by creatures. Large worms burrow through the stone leaving behind perfectly round tunnels. Dwarves set up mine shafts and tunnels. Troglodytes will even resort to digging out their own cave with adjoining tunnels after a split or fight amongst their previous tribe.

Ecosystem Cycles

y

With no sunlight, visible sky, seasons, or weather in the Underdark, most travelers can only track the passage of time based on their periods of rest. Most Underdark creatures do the same (if they care about timekeeping at all), unless there is a local means of keeping time.

Localized Phenomena

HAZARDS & PHENOMENA

Since its structure and environment is similar to the Plane of Earth, the Underdark experiences comparable natural phenomena, such as cave-ins, though some are more unique due to the denizens of the Hollow World.
Collapse When a collapse occurs, roll on the below table to determine the type of earth that fills the area. Creatures in the collapsed area must make a DC 15 Dexterity saving throw, suffering the listed bludgeoning damage on a failure along with being completely buried. A successful save results in only half damage along with being restrained by the rubble.
1d10 Type of Earth Damage 1-3 Soil 14 (4d6) 4-6 Very soft rock 21 (6d6) 7-8 Soft rock 28 (8d6) 9 Hard rock 35 (10d6) 10 Very hard rock 52 (15d6) Creatures buried must dig their way out, and they are buried beneath 1d10 feet of earth. Suffocation begins immediately unless the character can hold their breath or otherwise function without breathing. A character that is only restrained by the rubble can remove themselves with an action to make a Strength (Athletics) check against the digging DC of the earth type restraining them.
When a collapse occurs, roll for a random encounter to see what else might come through as a result of the opening. After a collapse the area is considered difficult terrain for movement purposes.
Gas Leak The adventurers come upon a cavern with a dangerous natural gas leak. Any member of the party with a passive Perception of 14 or higher detects signs of the gas. The characters' travel pace for the day is slowed by half as they circumvent the area, but there is no ill effects. If the gas goes undetected, each character in the area must make a DC 14 Constitution saving throw, taking 11 (2d10) poison damage on a failed save, or half as much damage on a successful one. Any open flames brought into the area cause the gas to explode. Each creature in the explosion must make a DC 15 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.
Lava Swell Occasionally earthquakes will occur that cause veins of lava to be displaced and move through any available cracks or a hole to open up into a lava vein. A tremor opens up a lava-filled fissure behind the party. Each creature must make a DC 12 Dexterity saving throw to avoid the lava swell, taking 21 (6d6) fire damage on a failed save.
Webs Giant spiders weave thick, sticky webs across passages and at the bottom of pits to snare prey. These web-filled areas are difficult terrain. Moreover, a creature entering a webbed area for the first time on a turn or starting its turn there must succeed on a DC 12 Dexterity saving throw or become restrained by the webs. A restrained creature can use its action to try to escape, doing so with a successful DC 12 Strength (Athletics) or Dexterity (Acrobatics) check.
Each 10-foot cube of giant webs has AC 10, 15 hit points, vulnerability to fire, and immunity to bludgeoning, piercing, poison, and psychic damage.
Random Encounter Tables The below tables can be used by the Dungeon Master as a source of inspiration when a party of characters is traveling around the Underdark.
1d100 Underdark Encounter 01-05 A drider cavestalker leading a party of driders 06-10 A troglodyte shaman searching for fungi 11-15 A mad drider babbling about fate and destiny 16-20 A hungry bulette 21-25 A skirmish between driders and troglodytes 26-30 Chitines from the Majestic Kingdom gathering resources 31-35 A ghost reenacting a scene from its former life 36-40 A ghost knight guarding a passage leading to Asphodel 41-45 A pile of corpses near a passage to Asphodel 46-50 A nest of giant spiders 51-55 A stone cursed from the Majestic Kingdom 56-60 Gargoyles looking to return to the Plane of Earth 61-65 A troglodyte hunting party looking for driders 66-70 A kobold colony from a distant land 71-75 A tunneling purple worm 76-80 An entrance to the Great Labyrinth 81-85 Collapse 86-90 Gas Leak 91-95 Lava Swell 96-99 Webs 00 The River Styx

Tourism

GETTING THERE

Reaching the Underdark is as simple as finding a cave that does not lead to a dead end before delving deep enough to connect to a cave system. The two best known locations to reach the Hollow World are the Mithral Mines and the Dead Falls. The dwarves of the mines dug a deep mine shaft to gather from the rich mithral veins found there. This great shaft is connected to a cave system that makes its way into the Underdark. At the Dead Falls, a waterfall has eroded a hole that leads all the way to the Nether Sea, the sides of which hold multiple openings into the Underdark.

SURVIVING

Finding sustenance in the Underdark is difficult but not impossible. While one can hunt, the best source of food can be found in mushrooms and other fungus. Mushrooms are abundant, but a traveler not trained in the ways of plants can easily fall to a poisonous fungus, believing it to be edible. Water can be found via underground rivers and lakes, though they should purified as they tend to be stagnant.

TRAVELING AROUND

Travel pace in the Underdark is significantly slower than for overland travel. Not only are the tunnels and passages often difficult terrain with uneven surfaces, but routes in the Underdark are rarely direct, and the travelers must follow available passages and their various twists and turns, climbs and descents. Creatures that can burrow through solid rock (such as purple worms) move at their normal burrowing speed, but this isn't likely an option for adventurers.

Type

Underground / Subterranean

Location under

Prime Material Plane