Spellsludge

Spellsludge dropped from the following mobs in the Underbelly in Dalaran:
 * Mana-Engorged Spiderling
 * Screek
 * Mana Seeper
 * Crazed Mage
 * Splint
 * Thieving Scoundrel
 * Segacedi
 * Xullorax
 * Underbelly Croc
 * Widowsister

As a quest objective

 * The Muck Stops Here