The Backrooms

Okay yeah, I completely get that.

Throwing him on to the floor

On to —> onto

McNamera noticed a beige shape moving towards him in a flash, along with a sharp pain in his back.

In a flash as an idiom is used awkwardly here and could just as easily be substituted with more appropriate words especially considering there is another sentence after it

at the attacker-

No need to cut the sentence off here, this is a complete sentence and a period is the correct option as a result.

Red blood; he was still a human.

The semi-colon is awkward here and is better substituted with an em dash. It's not wrong per se, but not the best choice

to lure out prey or groups of prey for consumption/assimilation.

To lure out prey for consumption/assimilation. No need to repeat the word if a shorter sentence does the trick

Each Shroud Taker possesses two forms in question

Each Shroud Taker possesses two forms:
There's no need to add 'in question' since this is the first time you're bringing it up

Due to their polymorphic nature, Shroud Takers do not possess any one "true" appearance. This is instead used to refer to any form the Shroud Taker takes aside from its disguise.

Direct contradiction. You can't say they have a true form, elaborate by saying they don't have one, and then go on to describe one. Either they don't have one or it's impossible to assume they have a concrete true form.

sustenance, due

No comma needed

Conversely, a Shroud Taker's

Conversely implies that this is an opposite. Hunting patterns have not been described for non-sapients.

suspicion by other members of the colony

by —> of

posses

Possess

atomically provide information on how to convert wanderers into Shroud Takers

Not a SPaG thing, but how exactly? How do they do this? Some future advice for you as a critic: If you see something like this, always ask the author how this happens. We cannot just assume something happens or state it as fact. There has to be theory or evidence.

systems; obtaining

; —> —

obtaining these cells is commonly done through the sharing of saliva from either spit, vomit, or sexual contact

Transfer of cells is achieved through oral or sexual contact with bodily fluids.

A Shroud Taker, when engaging in one of these ways, will produce saliva and atack cells via glands in their oral cavity and pharynx, delivering them into the other’s mouth, blood, or any open orifice

Here's an exercise for you as a critic: How could this sentence be improved? Ignoring the word 'atack' here, there is a very clear issue with 1) Underdeveloped pathway/attack pattern, 2) Unnecessary information (we already know how they transfer cells), and 3) Verbosity. This can easily be improved.

eat the wanderer alive from the inside in a quite painful manner

And people don't know this? How do the cells 'eat'? This is another thing you ought to look out for as a critic

have come to the U.E.C.'s database

Have been documented by the UEC. Stories don't 'come' to an entity. They are told, documented, brought to the attention of, but a story does not 'come'.

While first recorded in the 1600s, many have claimed to see individuals possessed by demons across the backrooms

Were they first recorded in the 1600s, or were reports of individuals possessed by demons first reported? Important clarification.

The earliest document was discovered in Level 11 and was dated to be written in 1609, and has been translated to fit within modern language:

Translated from what? This is not good worldbuilding if things exist just as things to drive the plot forward.

Many suspected him to have cannibalized them, though as I was able to find him traveling through the complex, I initiated conversation.

These two sentences should be separate and they do not have anything to do with each other unless you're describing aversion to talk to him because of the rumours.

What started as surprise seemed to brush away quickly as he realized it was me talking to him.

Though he was startled at first, he calmed down when he realised it was a familiar face that was talking to him. There's no need to try to make writing appear fluid through idioms if they don't fit.

Though the moments still were odd, as he stared at me very intently. Then his attention went to my features, trying to find anything strange. Yet he relaxed, satisfied without confirmation, and finally spoke to me. We talked for mixed moments about by business, and then his plans to move to one of the other camps. We did get to the topic of his friends, more or less by my own force. I don't recall what I asked him, but he did give a response.

He acted a bit odd at first, kept staring me down as if there was something wrong with me. I was a bit freaked out at first . "Since when was Charles this much of a prude?" I thought to myself. But eventually he did speak to me, about work, his plans to move elsewhere. We touched on the topic of his fallen compatriots, because I prodded around out of my own curiosity, and what he said chilled me to the bone.
See how much more fluently this reads? It's not even a polished draft, but if we're writing from the lens of the 1600s, this has to read like it was written in the 1600s. Not stiffly like it's currently written. I'm not critiquing the rest of this passage — I would have to completely rewrite it — but I'm just showing you that fluency is just as important as objective SPaG.

the wonders and horrors I have borne witness

borne —> bore. Also, avoid cliches like 'the wonders and horrors'. Just about every author writes this at some point, you don't need to keep hammering the point in that the backrooms is a mixed bag

purgatory: namely, Shroud Takers

Remove 'namely'.

At first things went well, but eventually the man in charge got power hungry.

Too sudden. This needs more buildup, the whole paragraph I mean

And when dawn rose

Remove and. As a general rule of thumb, don't start sentences like this with 'and'. This whole part is also telling us the exact same thing we saw in the previous paragraph. You can combine the two.

McNamera's guards were jumps

Jumps —> jumped

Stricken by their fear

Stricken by fear

soak the blade in red

Blades are not generally soaked, they are stained if we're talking about blood

lay on the floor with their corpses

laid on the floor, just corpses. They don't 'lay on the floor with their corpses'. They are the corpses.

but many, no color at all

but a sizeable portion of them did not seem to have bled human crimson. You can easily increase dramatic tension like this.

Every step I took, I felt a shiver down my spine. My body warning me that someone was still out there.

I shuddered with every step I took — every inch of my body screamed to me that I was not alone. In my heart, I knew there had to be someone, or something, just a few steps behind me.

There's a lot of other nitpicks that fall under 'How would Sariastuff write this' rather than actual objective critiques, but that comes with time and is a style of writing. If you critiqued this, these are some things you should look out for in the future.