[Richard G]

Hi Glenn,
thanks for the read.

The difficult part is the setting. The reference to Buccaneers and triangular trade (Trinity ships) suggests 17th or 18th century England,
Difficulty or not, you got there. To my mind it's early eighteenth (though you could simply view the whole thing as an alternate history and pick any date you like.)
but the references to extreme cold, (the Frost)
If I may direct you towards "Frost Fairs"?
diseases,
the descriptions are of diseases prevalent at the time (circa 1800. Similarly, the stench associate with slave ships.)
and the Shadow
I'm not sure it's an 'unspeakable evil' but perhaps the product of one.

Thanks again.

RG.