Perimeter-052 is regulated by Authority seafaring vessels HSC Valiant and DSV Endeavor on a biweekly basis.
Don't use the word "biweekly." It's ambiguous. Could mean every two weeks or twice a week. It's a terrible word and should disappear from the English language, so god damn useless.
the researcher must inoculate themselves with a 50mcg dosage of Oniritryptin viderics.
the researcher must compile their RPC-052-2 readings
I would make "researcher" plural so the grammar of "themselves" and "their" would be grammatically correct whether you believe in NuGrammer™ or not.
Documentation on RPC-052's people derives solely from secondhand reports made by researchers aboard a naval vessel, who had photographed RPC-052 on their way to the Faroe Islands.
I think this would read better if you stated the name of the vessel rather than just "a naval vessel" so that it's immediately clear that it was one group of researchers rather than multiple expeditions.
Between the years of 1887 to 1888
I think this would read better if it was "and" instead of "to." If you're worried about the grammatical bullshit of the word "between" and whether or not it can be assumed to be inclusive, I would swap it for "spanning."
Indeed, the inhabitants wrapped votive offerings of lamb, eel, and silver coins in baskets and flung them into the seaside.
I don't get why you're putting "indeed" at the start of the sentence. Seems like a break in clinical tone.
ascertained the exact location of RPC-052 had manifested.
however, after several weeks, no further developments were ascertained.
This second use of “ascertain” is in a different context to the first so I would use a different word here. It makes it seem like you’re talking about the same thing, the ascertainment from the first case.
seal and whale carcass
carcasses*
predictions of RPC-052-2's manifestations have been significantly difficult.
I think you meant RPC-052, not RPC-052-2 here.
We have received our orders. I and the crew are to travel from London to Shetland and finally, reach our destination at the shores of the Vanishing Isle.
This feels out of place at the end of the paragraph its in. I would move it to be the start of the following paragraph which it’s actually related to. Either that or make it its own paragraph. It feels like it’s tacked on unnaturally how it is currently.
This description is neither cruel nor unusual, however: the injustice is that he died like every other who was murdered in the wake of this terrible Isle, and I could do nothing but catalogue, record, research, move on. At least, until now.
Is it intentional that this paragraph doesn’t have a space between it and the last unlike the other paragraphs?
Disappearances are not an uncommon phenomena
phenomenon*
I am surrounded by old texts, and, I remain convinced that in them lie secrets
Why do you have commas before and after the “and”? It seems completely unneeded.
In the “Monday 27th February 1891” section, you jump between past and present tense a lot, and whether intentional or not, it reads kinda weirdly.
as fish is flung over the stage.
Should this be “as fish are flung”?