Crystal of Acidic Elemental Essence

Obtained: Random Loot Trophy off of Miasmas (good way to find them is to kill a Coral Golem Viceroy which has numerous miasma minions that all die when it does)

Uses:

Turn into Zahir of Stonehold for a 500,000 xp

(Zahir also gives the Elemental Eradicator kill task)