Is the reaper class the only one that can use this?
Nope, any class can. Just have to be level 80+
The monster that drops it is called Reaper King. It’s on 100f TOI which requires a 91f charm. And it’s currently impossible for anyone new to get a 91f charm. So only people who had 1 from a long time ago can farm that boss 
Incorrect. RKS is reaper only.
oh, i guess blank = reaper.
good to know.
Yep, Blank does = reaper, as reaper can equip any gear type