It's just a symbolic easter egg thing. Most mythologies/religions assign implicit effects for associating with certain forms of symbolism. Likewise, attempting to wield a tool in likeness of the reaper's own instrument ought to have the corresponding effect (only applies while in the 'abjected' state, though).As a side note, why does the Scythe that came from the reapers in the halloween moon bossrush freeze you? I don get it.
I thought this was something where you would sometimes find moving chests, and when you open it, the Reaper comes out of it to attack you
Also, I know you didn't want anything spoiler in the OP, so I will put this in a spoiler. I haven't seen it, but I am guessing that this is an enemy that is constantly moving slowly towards your position? And it will instantly kill you if it touches you. And it is possible to kill it, but it is extremely hard.
He doesn't just spawn at night, too... He spawns ANY time. And on a new character, It's ing terrifying.