A fixation on the concept of 'Epistolary fiction' sparked the use of this format, though some more general concepts I have had for some time inspired by Paul Shapera's Slenderman Opera are at play in the story, combined with some setting work for a friend's Monster of the Week game.
You can expect more of this universe and these characters in specific next month, as I have already begun work on a continuation of this narrative in a non-epistolary format.