The Grinder was instituted as a capital punishment for the worst murderers and criminals.
The Kayeed empire is known for it’s almost psychotic pursuit of justice and vengeance. This room was commissioned to deal with an rogue chieftain who had committed a long streak of atrocities against imperial citizens.
It is simply a room whose rough stone floor is continually moving - it is a huge stone roller driven by a paddlewheel. The stream powering the paddlewheel runs directly underneath the cell and keeps the cylinder ‘clean’. The joint between the moving floor and walls is knife thin, to prevent things from ‘jamming’. In any event, the massive roller that makes up the floor weighs many tons and supernatural force would be required to slow the roller.
Prisoners condemned to this room must walk. If they stop, they will be slowly, gradually be abraded by the moving floor. Death does not come quickly. Since the Grinder is self-cleaning, prisoners are generally never removed even after death. It is far simpler to leave them until their remains are totally ground to nothing, and a bit easier on the guards.
The other advantage of this means of execution, is that for worlds where magic is available to bring back the dead, death by the Grinder is very difficult to counteract. No intact corpse is available, and the sand-sized particles of flesh are washed downstream and scattered.
The most obvious one is to rescue someone important who has been condemned to death in this chamber. It will of course be heavily guarded, and the PCs may learn that there is a mechanism to drastically speed up the floors rotation…..
Although Ragaar the bloody met an end suitable to his name, even being ground to dust did erase him. Now his tortured soul, furious beyond the comprehension of the sane, wanders the dungeons around the chamber, bringing hideous death to those he finds. The PC’s are commissioned to stop his depredations, only to learn that maybe some tragic circles cannot be broken…