"At certain stages of the ritual they did grovelling obeisance, especially when he held above his head that abhorrent Necronomicon he had taken with him; and I shared all the obeisances because I had been summoned to this festival by the writings of my forefathers."
- "The Festival," H. P. Lovecraft
I had no deranged delusions, nor desire, to see the sights I saw. I had been on my way to the city, having been sent by my father, to secure a deal with one of the local merchants, a transaction of threads. If only he'd sent me any other day, on a fortuitous fortnight! But no, he sent me that day, which culminated in my doom.
The road I had chosen was clear, slippery with silence. The other option was packed with carts and with people. I thought I'd be quicker on the lesser-used road; it may have lacked the care, but I was on foot, and had no cart to lead, nor horse to ride. Thus I chose wrongly through logic, thus I brought myself to that feral festival.
The inn on that road was shabbily ran: after examining the bug-infested bed and the vomit-stained carpet, I rethought and decided that spreading my bedroll outside carried with it the promise of dignity.
It was late in the night when I was gazing into the fire I made. I was about to succumb to sleep when I heard a curious sound, a distant thumping. I, from my post at the edge of the woods, waited and watched, and saw a sight that made my curiosity flare. A procession of robed people, each bearing a torch, passed by the road. Clearly religious, I followed, wondering whether this procession was representative of the city. If it was, it might give me an edge in the negotiations.
Oh, how foolish I was! Cursed youth drove me, and corrupt curiosity pulled me, and I followed at a distance, and watched the proceedings.
They came into a clearing that had been prepared for the purpose. Channels previously dug formed some symbol that, if I looked down upon it from the sky, would shake my soul with its serene sin. I myself hid behind a tree, to watch the proceedings in morbid curiosity.
The group that I now knew to be some corrupt cult thrust their torches into the edges of the clearing, spaced evenly apart. Then the leader of the cult drew a dagger from some dark corner of his robe, and, to each cult-member in turn, cut into their hand and poured a couple drops of blood into the channels of the symbol. Though I knew it to be impossible, the pitiful amounts of blood grew in volume to fill all of the channel's depths of that depraved design. Some nefarious magic was at work.
Then the cult turned as one to me, each knowing my exact locale. I, paralyzed with both fear and their malevolent magics, could not run. Instead, the leader raised his hands, and I walked forward to them like a puppet on a string. One of the robed people produced a stake, another produced rope, and I was quickly bound to a spot in the middle of the symbol. As I looked around, looking for some way to free myself, I quickly learned something. The symbol carved into the ground- I was in its mouth.
But the cultists were not yet done. The leader stood behind me, and began to chant some immoral intonation. The rest of the cultists arranged themselves into various places on the nefarious image and began, in a most pagan fashion, an orgy in obeisance to whatever dark deity they worshiped. As the moments grew longer, the blood in the channels began to glow in a red light, and a fell wind blew through the place, silencing the torches, and still the cultists continued their demonic deeds, and the leader screamed His Name, a Name I had not heard before and refuses to leave my mind, its corrupted rot reeking into my mind and soul, a Name I dare not repeat here, in this account.
And then the jaws erupted from the earth, the demon the cults worshiped given flesh from their blood. Teeth forged of the red liquid began to close around me, to swallow me whole, and I looked down and the earth was fading into a dark, cavernous mouth and throat. Fear, abject terror gave me wings, the cult-leader's inept knot-tying gave me freedom, and I ran, right through the not-quite-solidified blood-flesh of the demon, I just kept going, running and running until I tripped over a tree root and I fell and succumbed into merciful unconsciousness.
I awoke in a room, with a bed and a blanket over me. A doctor came in, told me where they had found me, and tried to ask me a few questions. All I could utter was nonsense, my mind so tainted by that demon's Name that the words of my mind could not transfer to words from my mouth. But I can still write, and thus I write this.
May any god that may exist put a stop to that corrupt cult before that dark demon, that diabolical deity, puts a stop to them.
- Johnathan Crewes
"Out of the unimaginable blackness beyond the gangrenous glare of that cold flame, out of the Tartarean leagues through which that oily river rolled uncanny, unheard, and unsuspected, there flopped rhythmically a horde of tame, trained, hybrid winged things that no sound eye could ever wholly grasp, or sound brain ever wholly remember."
- "The Festival," H. P. Lovecraft
The world where Johnathan Crewes was driven insane has been kept purposefully vague. The purpose of this is to allow for assimilation into a multitude of worlds with minimal adaptation. It would be best in a world with magic, or gods, to allow for the demon to emerge, but in a sci-fi world the cult-leader could simply have hidden magnets or other tech to give his cult-members a show.
The cult worships the demon god for blessings. Any child conceived on the night of a festival- and to allow for lots of babies, there is one each month during the New Moon (so any moon gods cannot see what happens)- is gifted. This gift is not a supernatural or ostentatious gift, but a more subtle one. The child will be smarter and stronger than the average child- but also eviler, as the child is part demon. The children of these demon-children, if conceived during the Festival, will be smarter and stronger than the previous generation, and so on and so forth. The demon-god does this so that the cult, which grows as children and newcomers are initiated, can convert the world into a hell for the demon to inhabit and rule over.
The clearing with the marked channels is a well-guarded secret by the cult, and is watched over. The inn that Johnathan Crewes passed over is run by the cult, and used as a base to guard the clearing. As the demon-god needs a sacrifice to conduct the ritual and bless the children conceived during the festival, people discovered snooping around it are kidnapped and kept in the inn's basement until the festival. The clearing is also the only place where the demon can contact the world, which is why its so important to the cult.