I write about the devil a lot.
In the approximate center of a flat heath or bog, there is a tree. This tree is bent and coiled, growing down towards the ground then upwards again, strangled with a persistent leafless creeper that visibly grows from inside the tree. Its bark peels off in patches and dissolves under sunlight; its flowers are delicate pearly-pink and pop open from their buds every gentle night; its roots are deep and tangled and can be found all across the heath in knots piled over themselves.
Beneath this tree is The Devil.
The tree was recently planted as a flowering stake in The Devil's neck, pinning the creature into a hasty grave while it was being buried alive. The Devil is now broken up and scattered along the tree's roots in a dozen dozen forever-rotting pieces, leaking ambient sin instead of blood or lymph and getting waterlogged with acidified groundwater. The Devil is still very much alive, but immobile.
As a consequence of containing The Devil, the tree's seeds and trunk extrude a pungent vegetable oil if pressed down on; in fact, the whole tree’s interior is spongy, sodden, almost mushy. No seeds from the tree will grow anywhere, and branches cut from it will putrefy in hours, but the oil remains stable.
There is a family who live on the edge of this heath or bog, who go down to the tree and take its oil and mix it with caustic potash burned from the bog-salt, who make soap by the pound.
The soap is plain, scentless, vaguely blue, and lathers nicely when used. It has a mottled texture like large pores, or marrow.
Washing with it physically cleanses you from past sin: a whole layer of skin blisters, hardens, goes opaque and splits away from your body in a solid layer. Tiny, worm-like fibers on the inside of the skin writhe and die as they are exposed, physical filth crystalizes and freezes in bubbles on the surface.
The soap making family insists that these skins must be torn apart with sharpened salt and burned, as they will move of their own free will—notably, always back towards the tree, to become agents of The Devil and commune only as sin made physical can. One skin has already done this, endlessly scratching at the ground to break roots apart. The family calls it Toad, for its scuttling movements, cramped body, and the layers upon layers of cauliflower growths that cover it.