Blasphemous Cults Vol.1: The Order of the Feathered Lord (2007)

website blurb:
This document introduces a new an well-thought [out] cult for heroes to compete against. I personally used this as the backbone of a major campaign set on the waterways of the Empire. The first part of this document is written as by a Witch Hunter and can thus be given to players as a hand out if need arises.

user summary:
The first part of the supplement is a fictional diary of a witch hunter, Samael Neuhauser, describing the cult. The second part is the gamemaster's info, including a fully-developed cult consisting of a leader NPC and minions, a relic, a couple of brief encounters outlines, three adventure outlines, advice on players as cultists, and advice on running a campaign using the cult.

NB: According to the author, Vol. 2 of Blasphemous Cults is, within the lore of the set, an unfinished work. The Snail Man is the incomplete (in the game) volume 2.