And if that existential horror wasn't enough, it becomes clear that the perception filter works both ways, and humans no longer appear sapient to you. You can read the books in your local mouse library just fine, but the human road signs? Incomprehensible scribbles with no rhyme or reason. The humans are lumbering, unpredictable creatures which fashion large structures with bizarre, barely comprehensible purposes. They don't seem sapient, they seem monstrous. Just as wild as a mountain lion or an eagle, and just as threatening, yet their excess materials are strangely useful. It's terrifying. Every once in a while you manage to identify something with how it is in your human memories, you can extrapolate what the humans must be doing or saying because you remember what the human context is, but you cannot recognize human civilization anymore. Because you're a mouse now, living in a mouse's reality. And nobody else has been through this, so nobody else in this mouse world can understand what it is you're going through. And you're so small.

Happy Friday!

There you are sitting at your desk, maybe you're working longhand or your fingertips are tapping atop unpressed keys, and BAM! You have an idea that involves a monster that could've oozed its way right out of the Cthulhu Mythos.

Before you begin, pause a moment.

I get it. I like stories of the vast unknowable myself. I grew up playing Mass Effect and I'm particularly fond of the way Jason Pargin was able to nail it in his John Dies At The End series, and in such a way that I cared about the characters and their humors in spite of the overwhelming, multidimensional terrors that hunt them, but that's because I prefer heavily character driven stories and that's a diatribe for another day.

I've read a lot of aspiring fiction in this genre, and my main critique, the most common pitfall I see within cosmic horror, has nothing to do with character, setting, worldbuilding, or language. It has everything to do with writing that which is inherently unknowable, assuming you're trying to follow convention.

In other words: The monster has to be as alien to you as it is to the reader and characters. Forty page character sheets won't work here because at this point your "monster" isn't really a character. Remember, it isn't a being you can intelligently understand, and that's where the horror lives. It's a reckoning force defying nature, physics, and our fundamental understandings of science. Novels like The Three Body Problem by Cixin Lu illustrate this sense of scale and terror through sheer confusion and technological advancement.

Recall that Lovecraft's most popular story, The Call of Cthulhu, is epistolary. It's told through loose fragments, rumors, journal entries, it's never directly handled. Your job isn't to portray a gigantic, globular mass of eyes descending over New York City to deliver it's final judgement on humanity out of a thin blue Thursday afternoon. It should instead be the effect it has on the characters, or maybe second person to the reader itself, a virus in which just speaking or reading the name of your creature puts you at risk of harm.

One other issue I've come across in reading from a litany of fledgling unpublished fictioneers who take a stab at this genre is that it doesn't seem to be understood. The genre strongly echoes condemnation, damnation, the price of obsession, the price of knowledge, the price of ignorance, yes, but also the warning in bland optimism.

"Yeah, I'll just pledge my eternal soul to this unknowable deity 40,000 eons older than me, and then I will wield all the power."

That sounds dumb out of context, doesn't it?

It's not just about feeling earned or not, either. At this point, whether our earthly brother understands this or not, he's simply a vessel unbolting the latches of an old door sealed an unknowable amount of time before he existed. If we haven't been following him, haven't seen his transformation from upstanding citizen with a healthy few indelible and mortal sins to a hunched over, hooded lunatic who hides his deeds away from the very sun he orbits, this often lands flat and assumes stupidity on the part of your audience.

That's what makes this particular brand of horror so difficult, in my opinion. The balance from describing an unknowable, unfathomable monster that shifts through dimensions so as not to be physically described vs. making sure the audience knows that said impossible, indescribable force is destroying your character's mental state. Anyone can write, "I looked at the monster and it's very essence shattered my mind, scrambling it into a dark and forbidden wind, and even now trying to recall it sends shivers down my spine and vomit up my throat". It works. But it's flat without knowing who this character was beforehand. A slick talking lawyer bursting with personality? Okay, now we're getting somewhere.

So:

Before you start make sure

Your main character isn't your deity

Your main character is fleshed out well

Writing/reading is about the only time cosmic horror can work because it blends on disengaged senses. You're not really seeing, smelling, tasting, hearing, touching, but you are feeling. It's why hardly any games work in the genre without over explaining themselves or coming off cheesy, same with certain films in my opinion.

Leverage that.

Leverage Plato's allegory of the cave, your readers have only known shadows.