- For the daily quest version, see 
This should do the trick!
Those twisted hooligans are looking for souls to put in their crazy-juice? Then I say we help 'em out a bit!
Take a swig of this - don't worry, I've diluted it plenty... I'm almost positive that its only effect will be to detect any cultist's essence once you've defeated them.
When you've collected a bunch of them, throw the writhing mass of twisted souls into any of their cauldrons.
Let's see how they like a taste of their own medicine!
We have a few less cultists to worry about?
That'll teach 'em!