http://skyrim.melian.cc/?cmd=cmdSkyrimAlchemyWizard is awesome for trying out ingredient combinations in advance, in case you weren't familiar with it.
That's how I came to the Human Flesh/Imp Stool conclusion, though I'm not sure how that tool works when it lists the other items that aren't highlighted. Are there other ingredient combinations that produce the same effect? I saw a ridiculous poison on these boards the other day but, can't remember which thread.