Beauty and the Beast (Or, Come into My Parlour) - S3-E5
Trivia: While filming exterior scenes at "Spit and Polish" for this episode, the league had trouble with some members of the public. A car kept driving past them until they said "Action", and then it revved its engine so they had to start filming again.
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 ★