Modrons

The Modrons are the mechanical servants and soldiers of Mechanus, the realm of clockwork and machines.

March of the Modrons
There was a time, ages ago, where once an age the Modrons would descend upon the mortal realm and record and evaluate every single object of worth on the mortal plane. For most, this was an inconvenience, a modron would come into your house, look around, maybe open a few boxes or chests, and leave. For others however, it was more turbulent.

During the first few marches, banks would have their vaults smashed open, shops would have their doors broken down, and some caves would even be partially excavated, exposing secret places full of danger and ancient evils. Anyone who resisted was killed, whole bandit gangs dead by the modrons after they thought they could just fend them off; some towns lost most of their guard after a modron would kill an innocent who refused the modron entry, and the guards who try to protect the people not fully understanding.

After the first few marches, word spread that the modrons would not harm anyone who complied, and so shops and banks would just open their doors and vaults in order to help the march along. After several ages of this, once day - they just stopped. Shortly after, the Transmutation Bankers were founded, thus the suspicion that the Teller actually replaced the need to the Modron Marches, and thankfully so.