For reasons I will explain in due course, I'm trying to come up with a solid list of historical or semi-historical groups from the European Middle Ages (specifically no later than the mid-13th century) that might reasonably be called "secret societies." So, I'm thinking of groups like the Assassins or the Knights Templar, though, in a pinch, I'd probably be willing to accept purely fictitious groups like the Priory of Zion, since it's well enough established in nutcase conspiracy lore that to exclude it might almost require an explanation in itself.
Toss your suggestions into the comments and we'll see what we can come up with. Thanks!
EDIT: To clarify, I am not looking for groups that actually were conspiratorial secret societies in the Middle Ages. I am looking for groups that could be called such if one were of a mind to do so, as in, say, an over-the-top historical fantasy setting with demons and vampires and the like. The only historical accuracy I care about is whether a given organization actually existed or could have existed in the period, not whether they really worshiped a brazen head or were plotting the overthrow of Capetian dynasty.