If a patch of mold has sprung up in your cellar, you'd best investigate. Check the mold's consistency. If it's black, brown, green, or at all fuzzy, you're fine; scrub it off and don't worry about it again. If it's grey, streaked with pus-yellow veins, and spotted with 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 your brick cellar wall. Congratulations: an outgrowth of the Shimmeling, the mold of the Underworld, 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 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 oil 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. (Hope you weren't too fond of those clothes.) Keep shimmying along. 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.) 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.
The Shimmeling
The Shimmeling's massive guts are an ecosystem in their own right. They are host to an incredible variety of bulbous, luminescent fungi and wriggly, docile grubs. These creatures aid the organism in its digestion, which proceeds at an infinitesimal rate. Underworld worms wriggle freely in the slippery gizzard tubes, and lead 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 Underworld, they are perfectly sound within the tubes of the Shimmeling. A traveler in the Underworld may climb the slimy interior of the mold, the innards of which are akin to fleshy, ribbed, 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.)
Most notable among the Shimmeling's properties is its ability to create reliable gates between the Coast and the Underworld. These are not convenient passages, mind, given that they take the form of the Shimmeling's exploratory, probing vanguard of tentacles and sphincters. These vanguard probes are apt to emerge in the subterranean spaces of the World. In still, dead areas devoid of suitable food, the mold will cease its advances (though the creatures within its guts may not.) In areas rich with food (cellars, fungal caves) the Shimmeling will expand until it subsumes all available fuel, 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.