Dark and empowering presence. Some real influence from joining the faithful would help with recruitment...
Mental Poison is my runner-up, but we can probably get similar effects from things like deathrot, and maintaining a highly visible persona would stretch our resources. We couldn't go running off every time a goblin wanted to challenge us, and the costs of a lavish lifestyle would drain our limited financial assets... Mark of Carnifex is just like dark presence except it less likely to get caught, has less power, but works on everyone. Sort of the sure thing to Dark Presence's high potential but low reliability. I want Dark Presence over Mark of Carnifex because dark presence sounds like more fun.
I am not really opposed to initiation rituals. They give a sense of commitment(folk may well assume that some magic is going to bite them if they renounce the faith) and could be used to build a sense of familiarity with our order. Maybe we could use the staff's rot powers to make mushroom groves, and new initiates would need to spend a day and two nights in the grove, living on mushrooms... Alternatively they could just go to a cult safe-house and get blind drunk with a bunch of other cultists... Some thing that isn't so much painful or dangerous, but requires trust and at least temporary commitment. But if it is just tie them down, cut them up, and have them partake of our pure essences then I am opposed to that sort of thing...