So what do all of these Arkat sects – what we broadly call the Stygian Heresy – have in common?
First and foremost they all claim to be the inheritors of esoteric knowledge derived from the personal spiritual experience of Arkat, that is above any orthodox teaching, tradition, or authority derived from Malkion.
Secondly, they all identify Gbaji the Deceiver as the great moral evil of the world. Arkat’s struggle with Gbaji is the source of his spiritual authority (which all agree is of a superior authority to Malkion or others).
Beyond that, the sects are all over the map. Some sects openly display themselves. However, most hide and pretend to be orthodox Malkioni (especially Rokari) or are initiates of other local cults. As one hidden Archon is cited – “if any religion is stronger than you, follow it, but keep the secret truth in your hearts.” Many sects permit or even demand that members deny their membership to outsiders – such dissimulation was required for the survival of the sect during periods of persecution and have been maintained. Many sects forbid marriage outside the sect.
The term “Stygian Heresy” is of course not what they call themselves, but it connotes not only Arkat’s associations with Darkness, but the sects’ claims to have secret wisdom and esoteric lineages that they keep hidden “in the Stygian dark.” It does not necessarily mean that the sect worships the Gods of Darkness, although many do (but others worship Lightbringers, in particular Orlanth and Humakt, and still others claim to be Hrestoli).
See also
Does Stygianism have a foothold outside Safelster? there are likely many cells in Tanisor. Likely many of the Trader Princes subscribe to something based on Stygianism. There are cells in Nochet, and there is a Stygian stronghold in northern Esrolia.
Is Henotheism a Stygian Sect? Henotheism is an attribute of many different sects.