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.