Underground
An infinitely expanding plane of sprawling underground caves and caverns. In the center, a pillar wide enough to house a
whole royal palace, and the lands around growing to house billions of indivdiuals. It's an afterlife, a new start or simply a home.
All who live here are functionally immortal, barring some accident, and don't seem to age. The lands do seem to have a way of slowly morphing and mutating folks, though. Often for the better, but rarely can it give a fate worse than dying.
'Underground'is my oldest worldbuilding project, started in my head in about 2019/2020, and only put onto paper to solidify it in 2021. It's obviously inspired by my long interest in various shows such as Lucifer and Supernatural, and some bits of fun headcanon and fanfiction from other medias.
The World
History
Hell was, as of a few thousand years ago, almost entirely uninhabited. That wasn't to say it was entirely devoid of life, souls did occasionally arrive there, slipping through cracks to accidentally find themselves there. However, none of these folks are still around, at least they're not themselves anymore. The hundreds of years of solitude morphed them into something else.
When the first archangel did take it upon himself to discover where these souls were going to, and eventually located this hidden, secret plane, he made the decision to forgo his life in his homeworld to start his own empire somewhere else. He didn't want his work to be meddled with by his siblings, however, so he almost completely shut down the plane, putting a barrier up that his kind could not typically pass through without a certain key. Of which none of them had access to.
Over the years, he built up the world, bringing those who wandered aimlessly together to build towns and cities. He bestowed a small group with a bit of his power and named them the first Grand Dukes, leaders over various regions. However, this didn't last, the power was too much for them to take and over the next few hundred years, they slowly passed, leaving their regions to their heirs.
Climate and Nature
Like the common understanding in culture, Hell is generally quite warm, significantly warmer than Earth. However, it has evolved its own wide variety of biomes, plants, and landscapes.
The center regions are more barren, with the occasional tree and grasslands, though it's primarily rocky. Other regions are made up of great chasms spanning dozens of meters down into magma rivers. Other areas are dense jungles and forests, trees and foliage in shades of blue, purple and red instead of the common greens on Earth.
Hell is lit up on roughly a 48-hour cycle by groups of crystals that are found throughout Hell, regardless of where you are. They're usually embedded into the stone high above and in stalactites/stalagmites and use the ambient energy that's active throughout the whole plane the charge. They continue to glow even when harvested, so many use them for automatic lanterns, lights and torches for exploring darker areas during the day.
Changes
Hell has a strange aura and energy to it, causing odd things to happen to people who arrive. Souls who appear tend to start out looking completely human as they did on Earth, however over the course of a few years, they develop demon-like features. This change can sometimes be painful, especially when growing new horns, a tail or having your entire lower-half anatomy change, and it can be sometimes hard to get used to, but most eventually grow accustomed to it. These are considered a positive result.
The negative result either comes from an unknown 'illness', or from being alone and aimless for too long, not unlike those who arrived before the plane of existence was discovered. The illness happens seemingly at random, and can cause a soul to become restless and grow and morph into something truly terrible, a creature fitting of the name 'demon'. This form comes with an intense urge to leave, to escape Hell to return to Earth, which some do manage. Over a short time, they'll grow into monstrous creatures, black fur and white glowing insides below the fur. They're almost feral, wild creatures who will do anything and hurt anyone to get where they feel they need to go.
For those who wandered before Hell was discovered, they're mostly still around, just hiding in the wilds. The majority of them are completely wild, four-legged creatures. They take different forms, and can be many sizes between a dog and an elephant. They have the same features and look as the above 'illness' form of this, with fully black fur and white insides and features.