Chief Engineer Copperclaw aboard Orgrim's Hammer wants you to throw a Writhing Mass into a cauldron at Aldur'thar.
1x Writhing Mass Banished
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... Its only effect will be to detect any cultist's essence once you've defeated them.
When you've collected a bunch of essences, throw the writhing mass of twisted souls into any of their cauldrons.
Let's see how they like a taste of their own medicine!