The Kerberoi (singular Kerberos) are the arbiters and generally masters of the various Dominions within the lower depths of the Underworld. Generally, they only communicate with someone when this person has violated an Old Law.
The Kerberoi are not exactly rulers of the various Dominions of the Underworld; it is, perhaps, more accurate to call them extensions of those Dominions. Each Kerberos is a manifestation of the character of a particular Dominion, and of that Dominion’s Old Laws. It is in the latter state, as an enforcer of the Old Laws, that Kerberoi most often manifest as enemies to Sin-Eaters. Within each Dominion, the Old Laws are arcane, arbitrary, and often hidden. These qualities, though, do not prevent the Kerberoi from enforcing them tirelessly and with abandon. Often, a Sin-Eater’s first trip to one of the Lower Mysteries becomes an exercise in fleeing the associated Kerberos, as it is all too easy to inadvertently break one of the Old Laws and thereby attract the unwanted notice of the Kerberos. Fighting a Kerberos is one of the most foolish things a Sin-Eater can attempt to do.
Kerberoi cannot leave the Underworld. Punishments they enforce do not always include death, but they are always unpleasant. Sometimes, the lawbreaker will have to do a service for the Kerberos, sometimes they will be imprisoned or enslaved for a period of time, sometimes they will be immediately thrown out. Getting thrown out of the Dominion is considered the lightest punishment available, and not many Kerberoi are that lenient.
Kerberoi do not have a uniform appearance. Like very ancient ghosts or particularly strange geists, Kerberoi usually manifest humanoid shapes, but often with strange geometries, bizarre growths, or symbolic attachments and appendages. A Kerberos might appear as an Egyptian death-god, with a gray-skinned humanoid body and animalistic head, or as a spider-legged creature with four faces; most of them are decidedly inhuman. Along with their frightening countenances, they often have a wide range of Manifestations at their disposal. Many Kerberoi are also quite a bit larger than the average human, which lends them perceived weight and authority in the Underworld along with actual superiority in confrontations. Sin-Eaters who encounter a Kerberos had best be wary, for the creature rarely appears except when it undertakes the business of enforcing the Old Laws.