Something piqued my interest while finishing up my article on the pilgrims of the Flanaess. Of all the 60 deities of Greater, Lesser, or Demi- status listed in the original Guide to the World of Greyhawk, there is only one of Lawful Evil alignment; Hextor. One would expect, even with the loading of so many of the Greyhawk deities to the Neutral alignment, there would be more than a single being representing the ethos of both law and evil. I suspect this was not an oversight, but another sideways glance into the vast tableau that stretches beneath the surface of the World of Greyhawk.
We are told, in the Gord the Rogue short story "Cat or Pigeon?" (presented in the book Night Arrant, now lamentably out of print), that the "cult of Asmodeus" is a well-known source of evil doings, at least within the precincts of the city of Greyhawk. Within the precincts of the Horned Society, we are told that "deviltry" is the state religion (itself a change from the novels, where it is implied that the Horned Society serves the Daemons of Hades rather than the Devils of the Hells).
I submit that a cult dedicated to the Dukes of Hell is widespread in the Flanaess, and that most of the human (and possibly demihuman and humanoid) population of Lawful Evil alignment adheres to that faith. This cult would be seen as a great rival to that of Hextor; bear in mind that Hextor dwells on the plane of Acheron, rather than the Nine Hells.
Owing to the nature of their alignment, it would be a very regimented faith. All honor would eventually flow up to Asmodeus, but this could easily be done through intermediaries. Either individual sects would honor different arch-devils, or different unholy days along the infernal calendar would mark celebrations in honor of those lesser, but still puissant, beings. It could be a combination of both, of course, but in either case the chain of command, and the power of the sacrifices, would ultimately flow to the Arch-fiend.