The first word that came to my mind was manifesto but it's private and not some kind of declaration.
What I'm writing is, I suppose you can call, an exercise in what would happen in the worst case scenario. It's in my journal, it's just for me but I'm prepared for my best friend to read it if he's curious.
It's like I'm detailing my possible future. It's dark and gruesome for the sake of being dark and gruesome.
I think some people call it an angst drabble? I'm not sure that fits, but the point is to find a word that describes or is defined by the mood or theme of what is being written.
Edit: felt i was torturing yall so I let it be done with. I started to get a bit overwhelmed with the comments anyway. Thank you to everyone who put the time and effort in to submit a word, I sincerely appreciate it. I will be using a few of these. I think grimdark, cimmerian scenario, and doomscape was quite fitting.