Jacimus is a mysterious fungus from a meta-dimensional reality, and has established itself on an isolated volcanic island.
The mycelium network has carved its way downwards, into the now extinct volcano's core, crumbling the rock into a nutrient rich soil called Purpul Durt, where the whole place gets its name. For millions of years, an underground chamber has formed, hosting an entire fungal forest in its belly, spanning three hundred cubic miles.
The mushroom above ground is small, given how much work it does. Its color depends on the visual capacity of the viewer, but most can agree, that it has a luminous iridescence. This single mushroom gathers all the energy it can and delivers it to the underground kingdom, providing photons for its multitude of lifeforms. The mycelium covering the walls gives off a bright cyan glow, mimicking the outside sky. Other light sources come from the lifeforms themselves.
The mushrooms inside Purpul Durt are all one lifeform expressing itself in different ways, ranging from gigantic tree-like stalks with wide canopies, down to the tiny buried truffles. But Purpul Durt is not a closed system and other lifeforms somehow found their way in and have made it their home.
For millions of years, dinosaurs, small mammals, groups of apes, and lots of bugs inhabit the continent of URT. A meteor shower hits the southern hemisphere, turning it into a fireball. The sentient bipedal reptilian, the Squamaloid, having just discovered fire and language, retreat underground to the deep caverns of the planet. They begin to mine ores of incredible power, advancing technology to industrial machinery, forming hierarchies of leaders and workers.
The group of apes native to the northern most tundra planes would survive this catyclismic event their own way, eventually becoming humans called Disova. As the climate began to become friendlier, the disovans travel down into the low forested valley and start small villages.
A group of Disova have gathered around a campfire under the stars, roasting their rabbits caught earlier that day. A child has been designated to turn the rotisserie stick while the others laugh and tell stories. An elder old man, sits back at the cave entrance, just some distance from the campfire. He's meditating, chanting mantras about Irt, their word for the world where they live. He gives thanks to Irt, for the rabbits, and for the glue of the tribe.
The old man focuses on his senses; the sound of distant laughter, the crackle of the fire, the smell of the burning tinder and roasted rabbit. A breeze picks up from behind him. "Odd" he thought, but continued in his trance. Then it was more than a breeze, and the smell of burning, but not the fire. When he opened his eyes there was a bright yellow light coming from deep within the cave. Brighter and brighter it got until it was just as mid-day. The laughter stopped and the group came over to the cave, worried. A woman helped the elder to his feet in a hurry, rushing to get him outside. Not moments later, the yellow light got even brighter, filled the entire campsite.
A golden beast of stone and metal emerged, tearing the cave walls apart in its exit. Its feet were clawed wheels. A hatch opened up and out came a dozen large green creatures with nasty slimy teeth, three fingered tentacle claws, yellow eyes.
Squamaloid attempt to communicate with the people, but this is unsuccessfull. They decide that most disovan are not worth the effort and return to their subterrainian life. Occasionally, an intelligent disovan will not show fear and the squamaloid will invite it to join them. These disovan's are considered traitors among the people.
On their own, disovans build a society in accordance with nature and magick. An age of discovery and peace. The 'lizards' are declared the natural enemy of disovan kind. Defensive weapons and walls are built for 'when the lizards need our sun'.