Retest Now

STEP 1: Go to 49, 33 and drink the Diluted Cult Tonic. You will gain a buff called Dark Discernment.

STEP 2: Kill any mobs in the area and loot Tainted Essence x10

STEP 3: Once ten are collected combine them to make a Writhing Mass.

STEP 4: Toss the writhing mass into any of the three cauldrons. Each one will give a unique 2 hour buff (Spell, Attk, or Stam) as an added bonus.

Quest Complete.

Buffs last only in Icecrown.

here are links to those buffs:

Agony of the Fallen Cultist, which comes from the blue cauldron

Blood of the Fallen Cultist, which comes from the green cauldron

Fury of the Fallen Cultist, which comes from the dark grayish cauldron

one other thing, by waiting to deposit the Writhing Mass until i had both one Writhing Mass and 10 Tainted Essence in my bags, i was able to get both the stam and the spellpower buffs at the same time.

and, once you have the quest registered as completed (as in listed as complete in the quest log), won't let you collect any more Tainted Essence for the day.