Zahir

Zahir is a white robed NPC in Stonehold that gives a kill task requring you to kill 25 elementals for 5 million xp. He also accepts the crystals certain elements drop for 500,000 xp each.

There are four crystals (one for each elemental type):
Crystal of Acidic Elemental Essence
Crystal of Fiery Elemental Essence
Crystal of Lightning Elemental Essence
Crystal of Frost Elemental Essence

Zahir tells you, "Hello, you look like the very kind of person who would be able to assist me in my research." Zahir tells you, "After the events with the Harbinger and the Essences interest in elementals has gone up considerably and thanks to that I have been able to secure grants that I can use to fund my research... but you don't care to hear much more about that, do you?"
Zahir tells you, "Okay, here is what I'd like of you, Please venture out and find Caustic, Synnast, Inferno, and Hyem. Fight twenty-five of these creatures and then return to me and tell me everything you learned about them."
Zahir tells you, "I will also accept, and reward you for, anything you find that may be of value to my research."

You have killed 25 Caustics! Your task is complete!
Zahir listens to all that you have to tell him about your encounters with the elementals.
Zahir tells you, "Very interesting, I hope I can use this information to further my research."
Zahir tells you, "Oh, did you by any chance find anything in your adventures? I could certainly use materials that come directly from these elementals."
You've earned 5,000,000 experience.
You have been given the title Elemental Eradicator.