These creatures are automatons of bamboo and ceramic. Whenever they appear they enforce oaths and promises.
When King Thoukydides, secretly called Milk-of-Corpses, swore that he would never practice sorcery, it is said that an Unreliable Guardian would interrupt every ritual he attempted to perform, until finally the king hurled himself from the battlements of his castle. Yet they are equally likely to appear when a parent has declared that "I'll strangle that kid if he doesn't start behaving", and force them to do so, or to to imprison someone who has said "I'll never get out of here" until they starve to death, then seal the remains in glass so that they may not be removed.
It is unknown how they choose which broken promises to mend. They seem to have no preference for great matters over petty ones, nor good over evil. Nor do they treat offhand promises any differently to those bound with solemn ritual. It is, however, believed that they survey the world from on high. They have never been known to enforce oaths made on cloudy days, or indoors.
Upon completion of their mission, the creatures inevitably explode, spreading deadly shrapnel over a wide area. They will find an isolated spot to die if able, but if not they seem to have no hesitation in endangering others. They destroy themselves so thoroughly that no clue to their construction has ever been found. A blob of material rather like honeycomb has often been found in the wreckage. It is speculated that this is the creature's heart, brain, or soul, or that the 'creature' is but a vehicle, and this material is the corpse of the driver.