A creature for Trail of Cthulhu.
The shunned tribal elders called it Giaouchatnhon when they discovered it’s lair deep within the caves in what is now Vietnam. They saw the effects it had on the first unfortunates to be exposed to it, infected by it, and they studied the results.
The victims grew ill, pale and weak. They began to suffer delusions and terrifying visions of worlds and lives utterly beyond their comprehension. The elders recorded the babbling of the victims, restrained them when they tried in their delirium to seek out dark hiding places of their own, and after the inevitable final moment when the doomed man or woman collapsed into greasy grey dust they sifted through the powdery remains and they found the slug-like larva and roasted it over a flame.
The charred thing, ingested, would grant visions to the elder who consumed it and grant them knowledge and magical gifts that helped the elders cement their rule over the tribe.
But power is never enough power. The elders sent more and more victims into the caves to expose them to the spores of Giaouchatnhon, to sicken them with alien flesh growing within them, to record their delirious cries and then to consume the thing that had slain them. The elders became priests of a sort, and they consumed the larvae more frequently, becoming themselves other than human. And inevitably their untainted neighbours eventually decided enough was enough. The elders and their followers were executed and the sacred caves became the forbidden caves, and the entrances were blocked with rocks and earth.
Alhazred encountered it in his dream-wanderings beneath a brittle sky, a vast coiled worm or grub with an oil-black membrane and hardened roots at its extremities burrowed into the rocky wasteland beneath it. He wrote of plunging his hands through that crusty skin into the gelid interior and he touched its mind… its minds… its mind conscious at every moment of every place, every world, every age in which it grew and festered. One being, one consciousness, a thousand thousand dark places. He licked his dream-fingers clean and tasted each one of those worlds and he wrote down what he tasted. He tasted of darkness and the desire to spread itself wide across every world that was, of the desire to be carried to every speck of dust that made up the cosmos until it was everywhere and everywhen. He knew from the slime trickling down his throat in that dream that the creature lurked within the deepness of his own world too and he wrote down cryptic warnings about digging too deeply.
The Mi-Go recognise the signs of infestation in their own kind and in the bodies of other species they encounter as they surge from world to world. They burn the worlds that are too badly infested. They vivisect the victims, even among themselves, to study the growing infection and then they destroy the larvae with strong acid. Their symbol for the creature placed upon a boundary place will dissuade them from entering, as a red plague-cross on the doorway of a house deters visitors.
Behind The Scenes –
Giaouchatnhon is the name it was given on Earth by the tribal elders who first discovered it. It exists in only one location on Earth – for now – but it is the same entity that lives in and on countless other worlds across the cosmos. Every instance of it shares the same consciousness and it perceives at once every sensory experience of every one of its manifestations.
Its thoughts, you can be sure, are not human thoughts.
The life cycle of the creature is one of contagion and dispersal. The adult form resembles a vast slug, several metres long, with a hardened carapace and rigid talon-like roots at either end with which it anchors itself into whatever surface it has chosen for its home. While capable of uprooting itself and moving it does so very rarely, and it is clumsy and slow. It prefers to remain stationary since once it has put down its roots it begins to spread trails of filaments through the surfaces it is in contact with, thin lines of mold that can permeate through almost any material, forming a nearly invisible web around it and extending from it over increasing distances.
The filaments spread only slowly in daylight, faster in total darkness. The creature perceives the world around it through this web and the longer it remains in place the wider the net grows.
When the creature perceives a potential host moving within the compass of that web it will begin to form tiny nut-like cysts on the filaments. These will detach from the web when they are ripe, and about the size of acorns. Movement triggers them to burst releasing a small cloud of sharp spores which can be breathed in (causing irritation of the throat and lungs) or absorbed through any damp or broken skin.
The victim is now a host and the spores will invade the host’s cells like a virus and replicate themselves.
The host will become ill over the next week to ten days. At first a fever, tremors and night-sweats, then delirium. Their dreams will become disturbing with disjointed images and sensations, and inexplicable alien landscapes and appetites. After a few days those dreams will become waking hallucinations and the host will become helpless to resist the touch of the creature’s consciousness.
When the creature within him has multiplied enough the host will be compelled to seek out a dark place to lie down and rest. At this point the host’s body dissolves into greasy ash or dust and within that dessicated mass there will be the larva of the creature. No larger than a long thick finger, resembling a slug or black grub, it will wriggle to a place of safety and there begin to grow, and attach itself to the environment around it.
After a month it will be several feet long and will have developed enough to anchor itself in place and start to extend its filaments of perception around it. It will reach its full size after after six months and then it will be able to create its own cysts and spores.
Giaouchatnhon is a source of stories rather than a direct actor in them itself. Its earthly form is largely immobile, brooding within a deep sealed cavern beneath the hills of Vietnam, thinking the same thoughts as all its other manifestations across the cosmos, perceiving everything they perceive and waiting for a chance to spread.
Sooner or later those caves will be opened, perhaps to spelunkers, perhaps to tourists. Perhaps some reference in Alhazred's allusive text will drive someone foolhardy to explore in just the right place. They will become infected with the spores of the creature and carry them outside for the first time in countless centuries and Giaouchatnhon will have an opportunity to propogate itself.
Player characters may be part of that foolhardy expedition and companions of theirs may be the first to fall prey to the mysterious illness from the caves, growing sickly, their minds starting to crumble as they ramble about alien landscapes and secrets undreamed of by sane minds. Or perhaps the victim may be a friend of theirs returning from some foreign trip and falling ill... or perhaps by the time the investigators touch this story things have already advanced, and Giaouchatnhon is also growing in some damp basement in Chicago, or the store room of some night-club in New Orleans or London... growing and spreading its filaments and forming spore-rich cysts ready for the next host.
It may be that some modern savant has realised what they are dealing with and, like the hated priests of ancient times is deliberately cultivating the infection of others in order to use them as oracles in their delirium, or as growing mediums for the vile larvae which, roasted and consumed, grant magical power and inhuman insights to the ingestor. Cults may grow up around such creatures as in older days, offering up the helpless and hapless to deliberate infection simply to obtain the larvae whose foul bodies transform the human monsters who devour them.
If the cult fails to allow some of the larvae to grow though, if the cult prevents Giaouchatnhon from spreading, then it may become aware. It is ancient and it is wise and its patience is not infinite. With effort it can retard the illness of the hosts who carry the spores and reach out with its vast mind to take direct physical control over them as their own grip on sanity weakens. Investigators may not only have the cultists of Giaouchatnhon to deal with but controlled hosts who seek to spread the contagion further even at the expense of the cultists who seek to control the creature and use it for their own ends. And as mentioned above the Mi-Go are well aware of the threat posed by Giaouchatnhon to worlds they have a use for. If the fungi from Yuggoth become aware of an infestation on Earth then the investigators may find themselves in the middle of a massive conflict between inhuman interests... or possibly in a strange alliance with the Mi-Go as the slightly lesser of two evils.