ok, i've established i am not spoiler free so:
i kind of called it. fyo's ability having a literal crime & punishment theme. as in you do the crime, and he enacts the punishment. what i wasn't expecting, was that it doesn't seem to take effect until after he's died. someone has to kill him, with intent. he has to be murdered for it to work. he can't die to poison, can't take his own life ( not that he has ever wanted to ). and in turn, he subsumes, or absorbs, the killer. he makes the one who killed him the next fyodor/dostoevsky.
like i feel it's so specific too. because even if you use someone else the way bram did, it still applies YOU as the one with the weapon. they are an extension of your will-- and his ability catches that!! so, his ability will then plant him in your body, where it then remolds / reshapes you, into him.
it's also important to note, i think also if... you poison him ??? it counts as murder so like ? the only reason it wouldn't of worked in the prison was because he willingly poisoned himself, as part of the game.
like this bitch makes himself one of the most loath some creatures in all of bsd, and you can't even kill him. or you're going to die and he's going to climb out of ur body like the rat that he is ughhh.