Earth Elemental Fungi
The Oni Hell is a Yokai realm. One of many "fissures" that may be windows to fragments of past ages, island realms in the rift or somewhere in the dreaming or pocket dimensions. They are not whole worlds. This much sages know. Imperial agents have rescued some, and made entry to attempt to discover how Oni and other Yokai raids might be stopped. They describe a land of stepped karst - a surface burned and baked by two suns, and swept by scorching winds. Sometimes blasted by pelting rains that sizzle on the flat plain of the fragments and cover everything in a humid vapor mist. The flat surfaces - hundreds, sometimes thousands of meters across- are supported by pillars thousands of meters tall. Some shorter platforms can be found below the surface. In the branched undersides dwell climbing and flying things. Far below is an icy ground layer with frozen pools and icy streams and still more monstrous creatures. The prisoners work mines and send water, ice, cold air, mined resources, as well as meat and bodies (living and dead) to the citadels on the surface.
Report of Imperial sages, University of Lanhou
What the human Imperials believe is stone is actually giant earth elemental fungi that have grown up over thousands of years.
The fungal caps form the surface of the realm as they crowd next to one another or crumble away. the branched and veined undersides of the caps provide hiding and nesting spots for other creatures of the realm. The pillars are the stalks and may be encrusted with mineral extrusions. In icy ground are the mycelial networks feeding on the minerals of the soil.
The Earth Elemental fungus has a consistency of stone. It grows slowly, taking thousands of years to reach up to the surface.
Ecology and Habitats
Debris rain down to the ground layer - waste and bodies, bits of debris and crumbled bits of the caps. Mixed with this are spores that settle and grow in the soil. Earth elemental bugs burrow and hunt, grabbing prey as they cruise just under the soil surface. Larger elemental dragons and serpents hunt and dwell in ground, moving in it like fish in water or humans in the air.