If a patch of mold has sprung up in your cellar, you'd best investigate.
Check the consistency: If it's black, brown, green, or at all fuzzy, you're fine—scrub it off; don't worry about it again. If it's pus-yellow, stroked with grey veins and starting the ends of slimy, wiggling flagella, you may be in a spot of trouble.
Scrape off the mold and see what's underneath. There'll be more of it underneath, sadly. It'll have eaten all the way through the mortar of the cellar wall. See that tunnel, that mycelial orifice? Alas: An outgrowth of the Shimmeling, the Mold Between Worlds, has reached its sticky fingers into your basement.
Get an axe or a sledgehammer. Bash down that bit of the wall. Pull the wreckage away, minding the yellow-green goo, and survey what lies beyond. A great sphincter of sorts. Yellow-grey, slimy, smelling of bleach. Sort of a round, fleshy mold-door right inside your home.
At this point, you've a choice. Either douse the thing in kerosene and light it on fire, or crawl inside it. Presuming you take the latter option, go ahead and squeeze in. Go elbows-first, so you can pry the sticky sphincter open. Wriggle inside. Take note of how immediately slippery-slimy you get. (You weren't too fond of those clothes?) Keep shimmying. The going will be dark, but there'll be light at the end. Eventually, you'll find your way into some sort of stomach, a chamber with lots of other tubes feeding off of it. There'll be slime up to your waist. Note how it tingles.
There will be little nodules hanging by tendrils from the ceiling, like gross fruit. These nodules glow, giving off just enough light to reveal that the stomach-chamber is lined with countless tiny, wiggling flagella. If you stand still for too long, these flagella will attach themselves to your flesh and suck it gently.
Make sure not to stand around too long, lest the flagella suck you dry of juices. Keep squeezing through the slimy jelly-depths of the Shimmeling. (You'll most likely be used to the smell, by now.) Follow the purple veins (the big ones.) Avoid the symbionts (they'll liquidize you.) Follow them all the way to the heart.
The heart will be one of many, an oozy clod of purple mycelium, throbbing with pulses of weird ichor. Still have that axe? Good. Give the heart a couple of good, sound whacks. Note how the beast reacts: it'll begin to shudder and contract. The walls will squeeze you. Flagella will start to spew some sort of yolk-like lubricant. You'll be squeezed out of the heart-chamber, propelled by the gushy peristalsis of the Shimmeling's flight instinct. Hold your breath, now: the trip may be long.
You'll be vomited out into your cellar, extruded from the guts. Get up. Notice how the moldy sphincter-thing has retracted after ejecting you? You've chased it off. Board up the hole in your wall, and hope the Shimmeling never braves your cellar again.
Note: Yours was a best-case scenario.
The Shimmeling
In the vast and alien reaches between words creep the oozy fingers of the Shimmeling: A fungal organism of inconceivable size and endless hunger. It pushes its slow arms through leagues of soil and stone and spacetime, consuming every iota of nutrition. It displaces square kilometers of stone, gradually filling its path with the twisted passages, sacculated halls, and cavernous chambers of its own slimy gizzards. It is a beast of unknowable size, driven by some slow and hungry intelligence.
The Shimmeling's massive guts are an ecosystem, populated and maintained by a multitude of symbiont species. Among them are:
- So-called leukocytes. Marching, militant, ready to reduce your cells to sticky nutrient goo with gouts of caustic lysis-juice.
- Spiny salt-eaters with slick, extensile arms, like jellyfish, groping endlessly, mindlessly for exposed flesh, eager to suck the nutrient salts from your very pores.
- Wriggly, docile, haustric grubs that aid the Mold in its digestion. They are mostly harmless, but they are everywhere, and you can never sleep nearby them, for they will wriggle into your lungs, lured by the carbon dioxide of your exhalations.
- Varied, horrible species of chthonic worms—transplants from the Underworld—wriggle freely in the Shimmeling's slippery gizzard tubes, leading an easy, parasitic existence.
Other, more intelligent creatures can be found within the Shimmeling. They are not inhabitants, but travelers.
Though space and direction have little bearing in the interstices between worlds, they are perfectly sound within the tubes of the Shimmeling. A traveler may trek the slimy interior of the mold, the innards of which are akin to fleshy, rugose, fungal caves. Ample (bleach scented) air is present in these tubes, and the digestive slime, though caustic, is slow acting (its action upon the skin yields merely a tingle.)
The Shimmeling's most notable property is its ability to create reliable gates between the Coast and other worlds—most commonly the Underworld. These are not convenient gates, mind, given that they appear seemingly semi-randomly, according to unknown, presumably specific conditions, and are maintained only in accordance with the Shimmeling's unknowable appetites; not to mention that traversing them is a nigh-unsurvivable trek through legions of militant symbiotes and leagues of organ-like, toxic climes.
Shimmeling vanguard probes are apt to emerge in the subterranean spaces of the World, establishing "salients" where the taking is good. In stagnant, dead areas, devoid of nutrition, the mold will cease its advances (though the symbiont creatures spilled from within its guts may not.) In areas rich with food (cellars, fungal caves,) the Shimmeling will expand its salient, taking on eligible life as symbiont species and consuming all available fuel. The salient expands until it exhausts available nutrition, or until it encounters sunlight. The grey, wet tendrils of the giant mold cannot abide sunlight, and (fortunately for the people of the coast) never venture above ground.
Author's Note
Somewhat in need of an update.