Ayyy welcome anyways, just that you are aware, these characters belong to:
Bungo Stray Dogs

This is a very good anime. However, this show is extremely dark has a lot of heavy topics, and is a 16+ type of show. With that in mind, a one-shot based on BSD will be on the darker side and often messed up. Some will even fall into dead dove don't eat territory.

If you haven't attached the series, I would recommend doing that first or watching a video of these two characters, Fyodor and Nikolai.

Yes, they are based on the real authors. Most, if not all, Bungo Stray Dogs characters are like this, and 80% of pairings are toxic in one way or another, fyolai isn't an expectation with it being heavily one-sided

Warning:
-terrorism
-Christianity
-homicide
-suicide
-disgusting character not being clean
-blood
-weapons
-torture
-spelling errors (I write this late at night or too early to be awake sometimes, so I might miss errors 😅)
-cannibalism
-forced cannibalism 
-manipulation
-clowns
-toxic relationships
-gore
-heavy alcohol use
- major character death
-resurrection
-body taken over^
-rats
-cursing
-dark themes
-child neglect

There's probably more, but I don't really remember right now, but enjoy!