Like all rare tagged weapons, the Grim Reaper can be found on any stage in Raid Mode, but there is a much higher probability of obtaining it by killing Wanted Enemies or playing The Ghost Ship / The Ghost Ship: Chaos.
It's also possible to get this weapon by obtaining high scores in some RE.NET Online Events.
The highest number of custom parts slots on the Grim Reaper is 5. In the PS4, Xbox One and Nintendo Switch versions, it's possible to find this weapon with 6 slots but this will only come from The Ghost Ship: Chaos.
Just like the rest of the rare tagged weapons, the player will move at a slower speed when the Grim Reaper is equipped.