Okay. You have some pretty significant areas for improvement here. I'll try to help

I'm focusing just on the prologue right now, but these tips will help with all your writing.
1. Figure out what tense you're writing in and stay there. I noticed you shifting tense a few times, which is jarring on the reader. You wrote, "
it is a complete and total void, where you cannot see what lied ahead of you. "Is" and "cannot" are present tense; "lied" is past tense. You should write "lies" instead to maintain present tense narrative. Or change "is" to "was" and "cannot" to "could not."
I held my finger on the trigger, ready for any moment when the demons will strike. Again, "held" is past tense, but "will strike" is present tense. It should be "hold" for present tense and "would strike" for past tense. Always make sure you're using correct tense.
2. Here's a big one. Work on eliminating your use of superfluous words. This piece contains a great deal of wordy explanations and prefaces, far more than necessary. Some examples of what I mean:
I had begun to become comfortable in my rhythm. Consider this instead, "I was comfortable in my rhythm." OR "I had grown comfortable in my rhythm." Really. Less is so much more. "Begun to become" is awkward sounding. Plus, "begun" is the past participle of "begin," and past participles are rarely needed in fiction. You are describing a scene that is happening to your character, not something that he had started to do awhile ago, right?
It was that moment when I yawned—that exact second in time—that the battle had begun "That moment" and "that exact second in time" are exactly what I mean by superfluous. They are clutter. They are unnecessary because it is already implied that what you have written is happening at "that moment." Readers know this already.
Every moment is an exact moment in time, right? Why not just write "The battle began."? Readers will appreciate it, I promise

More words are only better if those words are actually doing something to enhance your story.
they were in the wrong places, as if they had been placed randomly That's just reiterating.
We fired out bullets at once to the sound of the shrieks. Obviously, you fired bullets. Bullets are what come out of a gun, it would only be remarkable if your character possessed a gun that fired something other than bullets. Like plasma or tranq. darts. Don't waste words on the obvious. Also, just write "fired" instead of "fired out." The "out" part of the sentence is implied already with the word "fired." It would be the same as writing it this way: "We fired bullets. Bullets came out of our guns." Same thing with "at once." It is already implied. Everyone understands that projectiles are shot instantaneously from a weapon once the trigger is pulled. Here's a better sentence for you to copy and paste for free: "We fired at the sound of the shrieks." Simple. Stronger.
3. Work on your similes. A simile is when you compare one thing to something else, especially by using the words "like" or "as." It is a great and powerful way to paint a picture in the readers' minds. But it should be used sparingly, and ONLY when the simile is evocative. How many times have you read, "sank like a stone" in fiction? Too many, right? Be careful of dull or cliched similes because readers tend to skip right over these without it registering when the idea is to have to
opposite effect!
You wrote,
It started with a blast like a blast of thunder rumbling through the air. Thunder rumbling is pretty good, but comparing a blast to another blast is bad. Try something that actually makes readers think, or better yet,
feel the sensation you're trying to describe. It works best when you compare things that are actually dissimilar. Here's a few that I used in a story I wrote: "It was quiet but for the cawing of
scavenger birds that spread out like a blanket onto the purple night sky." And, "The meal was greasy and wonderful; it
sank like a warm sunset into the empty pit of his stomach." Be original!
4.
“It’s,” one said, at a loss for words, “repulsive.” This is just a bad sentence. Who is the "one" in this? You never specify. Consider: "'It's repulsive," someone said.'" If you're going to separate quotations with a speaker attribution in the middle, try to at least separate the independent clauses. Would you want to read a sentence such as: "'I think I,' he said as he was walking down the street, but before he encountered the lady in a red hat, "am going to buy some ice cream."? Probably not.
5. Don't use passive language; it's flat and boring. What I mean is this: stop writing things like
I could already hear more growling. and
I could see a body shaking and
I could hear the occasional rustle when you can simply make your verbs stronger by changing the tense. "I heard more growling," and "I saw a body shaking," and "I heard the occasional rustle." Readers prefer lines such as these. I encourage you to try this and see how much more forceful and interesting your writing will become. It's a very easy fix
6.
Be precise and clear! Be descriptive! and show, don't tell! You mention a "scouting vehicle" at least three times, but never describe it. How can I form a picture of what you mean? Is it a jeep, a sedan, a truck? Is it covered or open-top? Armored? Dingy and rusted or glossy and brand new? does it smell like car wax or leather interior, are the seats cushioned and saggy or hard plastic and stiff? Is it pistachio green or honey mustard yellow or what? etc, etc. Rather than repeat a generic term over and over, you should try to pick out a detail or two that will really stick out in the reader's mind. Something unusual rather than common. If I were going to describe a kitchen, do you think I'd say "It has a fridge and a stove and some cupboards"? No. I'd talk about how it smelled of disinfectant and had a bowl of overripe home-grown apples on the speckled marble counter-tops, etc.
Precision:
The air was filled with fear now. Air cannot be filled with fear. Air is air. Show us something scary rather than telling us that something is scary. Readers want to discover your characters and your world on their own. Let them make up their own minds when to be afraid. Instead of saying something like "John was scared." Tell us about the beads of sweat forming on his brow, or the way his hands tremble or how his breathing has grown more rapid. Trust your audience to figure it out. They will.
My heart was beating in my head One's heart does not and cannot beat in one's head. That is where our brains are. A person can, however, feel his pulse inside his head. Try to be clear, always.
I could hear their growls. Their clicking. Their movement. Their biding. "Biding" is not something that makes a sound. It cannot ever be heard. Other sounds can be heard that may indicate that someone or something is biding their time, etc. But you can't hear biding.
The light of my rifle was sharp on the nighttime No. "Nighttime" is not a tangible thing. It is, well, a time of day when there is no sun. Light can't shine on nighttime itself, it can only shine on the absence of light that occurs during nighttime.
“Thanks,” I said with an irremovable smile on my face. Irremovable means "not able to be removed." Literally. Consider a better word.
Good luck!
