Chief Engineer Boltwrench aboard the Skybreaker wants you to drink the Diluted Cult Tonic, kill any Cultists at Aldur'thar and take their Tainted Essences. You are then to combine 10 of the essences into a Writhing Mass and throw it into any of their cauldrons.
Those twisted cultists are looking for souls to put in their crazy-juice? Then let's 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 collected a bunch of 'em, throw the writhing mass of twisted souls in one of their cauldrons.
Let's see how they like a taste of their own medicine!