The Underworld

The Underworld (located on planet Hell) is the realm in which Lucifer and most of the demon population reside. The planet itself is a rocky planet roughly the size of Jupiter and resides within a solar system located in the andromeda galaxy. The planet is about 9.2 billion years old. 
== Topology and Speleology ==
The surface of Hell is barren, rocky, and holds little to no atmosphere due to the heat from its sun. Being the closest planet to its respective star, the surface superheats during day to about 2,000℃ (3,600℉) and cools to cryogenic temperatures at about -200℃ (-330℉) night due to the lack of an atmosphere.
Under the surface of the planet is where most of the activity takes place. Known as the underworld, a vast system of massive caverns covers the entire underbelly of the planet, with rocky pillars of varying sizes keeping the outer layer of the planet from collapsing in on itself. Much of the cave zone is covered in lava, forming seas of lava, and lava rivers and lava falls being fairly common sight. 
The core of the planet is an active liquid mass of 90% iridium, 9% cobalt, and less than 1% of other metals. The high density of the mostly iridium core makes up about 40-45% of the planet's total mass. This causes active plate tectonics, meaning tremors and earthquakes are common occurrences. Along with this the high density of the planet combined with its size creates an abnormally high gravitational force. While the atmosphere on the outside of the planet is nearly non-existent, the atmosphere in the underworld is quite thick, and the rocks that make up most of the landscape of the underworld are made up of a dense iridium-silicon mixture. This is especially true in the pillars that hold up the surface, which need to be innately stronger because of the overwhelmingly powerful gravitational pull.
== Geology and Ecology ==
The underworld has a diverse ecology with many differentiating biomes. Silicon-based lifeforms inhabit most of the land in the underworld while tungsten-based lifeforms inhabit the various lava flows across the planet.
=== Geological Phenomenon and Landscapes ===
The ever-changing tectonics of Hell create some very interesting geological biomes. Some of the documented landscapes can be found below.
==== Hydreliothermal Vents ====
Large gas deposits of hydreliox (a combination gas of hydrogen and helium) under the rocks near lava beaches combined with frequent quakes cause fissures in the ground allowing the gas to bubble out of vents in the ground. The thick atmosphere of the underworld along with the lightness of the gas causes it to shoot out quite fast. Some species have evolved to float up the "air elevators" and glide from the top of them to get to places and evade predators.
==== Fossil Crags ====
The ever-shifting tectonic plates causes large masses of rock to be pushed up vertically, creating vast areas of massive crags and cliff faces. These crags often contain an unusual number of fossils, specifically of tungsten-based life forms. This is because most tungsten-based creatures have strong shells, bones, etc. which fossilize extremely well. When the rock faces get pushed up and become crags, the fossils get pulled up with them. 
==== Pillar Forests ====
Large iridium-silicon rock pillars hold up the ceiling of the underworld and seem to concentrate in certain areas, hence the title of forest. The pillars vary greatly in size, ranging from mountain sized to tree sized. Scattered throughout the pillars are many lava rivers and lava falls as well. 
==== Obsidian Flats & Lava Seas ====
The lava flows from rivers eventually all collect in the lava seas strewn about the underworld. The lava seas are home to an intricate ecosystem of tungsten-based life, housing over 75% of the species on the planet. Being so thick and dense, most creatures living in the seas have evolved appendages more adept for digging and pushing rather than swimming. 
When a lava sea cools off or drains, long stretches of obsidian flats are formed. These flats are often rich in cobalt oxide, giving the obsidian a blue tinge creating shimmering plains. Because of how light reflects off of the obsidian, strange reflections of light similar to the Aurora Australis of Earth can be seen dancing on the ceiling. Quakes will occasionally pull parts of these obsidian flats into the fossil crags, building large crystal glass walls. 
==== Keisoushi Tropics ====
Keisoushi are a silicon-based lifeform that share many similarities to the carbon-based kingdom of fungi. Commonly known as Rubber Toadstools, they have vast speciation and grow in large numbers coating massive areas of the underworld. About 27% of the underworld is keisoushi tropics. These areas are also much cooler and have a thinner atmosphere, due to specific qualities of keisoushi. Keisoushi feed on thermally charged gasses in the air and release endothermic fluids, making it a hotspot for biodiversity. 
=== Ecology and Biodiversity ===
Hell is home to two very different types of life, being tungsten-based and silicon-based. This allows for extremely diverse species and ecosystems in the underworld. Some broad ecosystems and specific species have been documented below.
==== Aerokrill ====
Aerokrill are a type of silicon-based life that resemble krill on earth. Some species are more like a traditional krill shape and some more closely resemble crabs. Size varies between species as well with some being only a few centimeters to some reaching the size of an average house cat. Nearly all species live near hydreliothermal vents and have adapted to using them for escape methods. Aerokrill are aptly named for their ability to glide long distances using their shell like a paper glider. Their shell folds in on itself providing extra protection while foraging, when escape is needed, they rush for the nearest air elevator and unfurl their shell to cover a wide surface area; once the pressurized hydreliox gas pushes them high enough they push forward and glide away to safety. They are also able to glide from vent to vent to travel longer distances.
The smaller species that resemble krill more closely are often found in large shoals of thousands. Hopping from vent to vent in unison they can confuse predators by changing shape of the shoal or spreading out among separate vents. The more crab-like aerokrill are usually solitary and only get together during mating season or when large carcasses are found to feast on. 
==== Keisoushi ====
Keisoushi are a silicon-based lifeform that share many similarities to the carbon-based kingdom of fungi. Commonly known as Rubber Toadstools, they have vast speciation and grow in large numbers coating massive areas of the underworld. Keisoushi are a very unique lifeform in the fact that they feed off of the heat from nearby gasses in the air, because they are constantly feeding, they sustain the heat within their bodies and therefore remain warm to the touch. When they have absorbed all of the heat from the gasses, they release an endothermic fluid. The process of them removing the heat from their surroundings and releasing cooling liquids in large numbers effects the environment drastically, bringing the normally extreme temperatures down to tropical earth temperatures.
Some species of keisoushi are able to move freely using leg like appendages during juvenile stages, they use this to find a perfect spot to propagate. 
==== Stonewalla ====
The stonewalla are a species of tungsten-based lifeform that exist mostly in the pillar forests, specifically living near lava flows. Stonewalla live most of their lives in lava but will return to land during mating season. The eggs they lay look near indistinguishable from the rocks in the nearby environment. Hatching on land where the temperatures are less harsh than the lava, the juvenile stonewalla build up their immunities to the immense heat and pressure of lava overtime. When they are old enough to easily survive, they will find an empty spot in the lava flow, usually near other stonewalla. Stonewalla have very strong, rocky, tungsten heavy bodies. They use this fact along with their bulky bodies to wall up by a lava flow. Submerging their bodies, they open a mouth like vent on their head and begin filtering in the lava. Any microorganisms in the lava get filtered in and eaten while the rest of the lava is filtered back out. Since they nearly never move after planting themselves, they don't need much energy to survive, and many stonewalla get together in one area. 
The conglomeration of stonewalla often cause dams to form, forming new ecosystems that many other creatures live in. Only the stonewalla on the top walls return to land for breeding season as to not destroy the dams. Once a stonewallas lifecycle is over, they become a permanent part of the surrounding rock. Due to this the natural tungsten amount is much higher in regions where they live, and you are able to see many faces in the rock from the generations of stonewalla living in the area. 
== Archeology and Demonic influence ==
Demonic civilization has had a big impact on Hell and the underworld for billions of years. The first demons evolved on Hell roughly 2.5 billion years ago. These first demons were nothing like modern demons, as they didn't have any interaction with human souls. Up until about 300,000 years ago, demon society was peaceful. Most demon species had very low levels of natural determination. This caused them to not evolve much past small villages scattered about the underworld. 
After Lucifer discovered the potential in demon society, he began experimenting with them and human souls and spirits. After about 100,000 years he found a way to perfectly unify a demonic entity with human souls/spirits, giving demons the ability to use resolve. Within just a few millennia, demonic progression skyrocketed, with them now having complex technology and cities, putting them somewhere between a type 1 and type 2 civilization. Through Lucifer's influence the demons became very war focused, with military duty and war being the most important things to their people. They built military camp focused cities, mostly in the pillar forests, with long hyper highways connecting the distance in between them. Lucifer also owns a few large castles on Hell, one being on the surface, though it is unmanned due to the extremes of the environment. 
== Trivia ==
* Music for the underworld is often jazz focused, while specific demons get metal instead
* The biggest inspiration for Hell was the underworld from Terraria
* Before Lucifer's influence, the planet was named an kassian ki by the ancient demon races
category:Fate's Wish: Warrior of the Omniverse