If someone reaches the lower Mysteries of the Underworld, he becomes subjugated by the Old Laws.
The Old Laws are, in most cases, ancient dictates that pile up more and more when one descends deeper into the underworld. Each of the Dead Dominions has its own specific Laws. The Kerberoi act as enforcers and judges of the Laws and punish trespassers.
Mages trained in the Fate Arcana can attempt to use their magic to allow them to break the Old Laws without notice from the Kerberoi, but this is extremely difficult. Archmages are one of the few beings that are capable of easily violating the Old Laws and getting away with it. Archmages can defeat typical Kerberoi with their magic, but the deepest domains are ruled by the Deadlords, who are capable of at least matching their power.