Welcome To Royston Vasey - S1-E1
Visible crew/equipment: During the scene where Benjamin arrives at his aunt and uncle's house, there are a couple of shots in which you can see crew members in the reflection of the grandfather clock in the front room.
Chosen answer: The members of the League refuse to admit what it's actually supposed to be, but have stated that it's possible to work it out from clues in the series. Human flesh seems like the most likely candidate, probably supplied by Edward and Tubbs from their general butchery of any non-local who sets foot in the place. Another theory is that cocaine is involved in some way as its effects seem to match most of the things that the special stuff is supposed to cause. So, combining the two, and why not, the "special stuff" is non-local human meat sausages impregnated with cocaine.
Tailkinker ★