This is an evil vigilante group which exists to strike down powerful or influential non-evil men and women within Aerdy. Their prime targets are priests, especially those of Pholtus and St. Cuthbert.
The organization is generally thought to be in the service of Nerull, the Reaper, and their aim of bringing death to servants of good is fully in accord with that general view. Indeed, priests of Nerull are among their number, and the second echelon, the preceptors, is almost exclusively of that brethren.
Since the cult is only believed to have been active for 30 years, and its killings only linked to it for many fewer, no man has been able to collect a great deal of information about it.
The head of this hierarchical cult, a man known simply as “Hidden Sickle” to the preceptors, is actually a priest of Tharizdun, one Karnquiza. In brief, he is a servant of the Scarlet Brotherhood. While this is not especially surprising, what is perhaps startling is that the preceptors haven’t yet realized this.
This continuing ignorance suggests that, at the very least, The Reaper himself is happy to go along with this charade.