Obtain the four medallion fragments from Haalum, Eykenen, Lakaan and Uylaru and return to Arcanist Thelis at the Sanctum of the Stars in Shadowmoon Valley. Performing quests for the Scryers will cause your Aldor reputation level to decrease.
According to Akama's letter, four fragments of the medallion were unearthed and given to the Ashtongue Corruptors. These dreaded Broken were once powerful shaman who now defile the very elements they revered.
Haalum, Corruptor of Air, can be found at Netherwing Pass. Eykenen and Uylaru, Corruptors of Earth and Fire, were last seen at the Fel Pits near the Hand of Gul'dan. Lakaan, Corruptor of Water, is rumored to be near Coilskar Point.
Find them and obtain the medallion fragments.