Shedaklah

From Fiendish Codex I: Hordes of the Abyss (3.5e):

The 222nd layer of the Abyss is a bubbling morass of oozing, fetid sludge situated between two sluggish branches of the River Styx. Here and there, a low hillock or fungal forest peaks up from the plain of caustic, unintelligent slime. An overcast sky of mud-brown and rotten green clouds casts the layer in pallid gloom. Every few hours, a pregnant cloud bursts, spilling torrential rains of polluted water. A thin film of cloying moisture covers everything on the layer.

Shedaklah is the contested Abyssal domain of the demon lords Zuggtmoy, Queen of Fungi, and Juiblex, the Faceless One. Conflict over dominion of the layer persisted for centuries with Zuggtmoy retaining the upper hand, keeping Juiblex and his intelligent slimes to the deepest caverns and far from her fungal children. Recently, however, Zuggtmoy's Material Plane machinations with the demigod Iuz drew her from the Abyss, and ultimately into a magical trap below the infamous Temple of Elemental Evil. In her absence, the Faceless One surged throughout all of the subterranean reaches of the layer and even onto the surface itself. Since returning, Zuggtmoy's primary objective has been to drive the Faceless One's minions back below, but thus far she is meeting with frustratingly little success.

All manner of plant and fungal creatures dwell upon the surface of Shedaklah, paying honor to Zuggtmoy by means of the parasitic processes that keep them alive. Phantom fungi, myconids, shambling mounds, tendriculoses, and giant sundews—many fiendish or half-fiendish—abound on the layer, reveling in its unnaturally warm dampness. These faithful servants of the Fungi Queen seek out and destroy the few alien beings of organic ooze that emerge from the lightless depths of the layer, but in the deeper caves the slime creatures of Juiblex are so numerous that one must swim through them to make progress.

The listless souls of victims claimed by Zuggtmoy's cults on the Material World wander the surface of Shedaklah, numbly seeking shelter and hopelessly fishing for food that is too infested to eat. When these constantly starving, endlessly suffering playthings grow too sparse, the demon queen turns her attention once more to the mortal realm in an effort to increase the power and influence of her cult, bringing more and more lost souls to Shedaklah.

Due to its riotous abundance of life, the layer is infused with life-giving positive energy, reflected in its minor positive-dominant planar trait. All living creatures on Shedaklah gain fast healing 2 as an extraordinary ability for as long as they remain on the layer.