For nearly all of Time, The GreenMan faith served the people of The Land. For decades new Faiths made inroads among the people of The Land. The New Faiths came with the traders and a few invading armies. They were the “mainstream faiths” for much of the world. People saw weakness in their old ways (as invaders were coming in and setting up new homesteads and the people of The Land were powerless to stop them) and saw the “advantages” of the new Faiths. The new faiths also spread many rumors (and even some outright lies) about the Old Green Faith.
Some of those were quite unsavory.
A small contingent of the Old Green Faith broke with the passive submission of most of the Green Council of the Faith. If they, followers of the GreenMan, were going to be branded murderers (non voluntary human sacrifice) and thieves, then they will live up to and exceed the lies of the new Faiths.
The Dark Sickle uses the old green magics and forest crafts as practiced by the GreenMan faith to kill and steal. Over the decades they have adapted the greenways to city ways. In addition to attacking high ranking officials of the invaders and clergy of the New Faiths, they will take on other targets (on commission). The price is dependent upon the opposition the the target (or target’s owner) posses to the Sickles. So invaders, “collaborators”, active followers of the New Faiths, and nobles who have handed land over to the invaders, are all less expensive targets, while those faithful to the GreenMan faith are excessively/ prohibitively expensive.