Crystal of Acidic Elemental Essence
A crystal of Acidic Elemental Essence.
Value 0
Burden 1
Stack Size No
Obtained By Random Loot Trophy off of Caustics
Notes Turn into Zahir of Stonehold for a 500,000 xp

A good way to find them is to kill a Coral Golem Viceroy which has numerous Caustic minions that all die when it does and then the crystal may be in one of the resulting treasure piles.

The NPC Zahir also gives the Elemental Eradicator kill task.