Head back to ( -755, -52, 395 ) /waypoint -755, -52, 395 (if you had to wander off to find your elemental) and there should be an elemental with the title "The Wounded" under its name. It's a ^^^80 heroic mob. Use Query Element on the wounded earth elemental. Attempting to use Dominate Element on the wounded earth elemental results in a message stating the "element is solid, and will not allow you to dominate it." Hail it to get it to aggro. (This is intended.)
Kill it and retrieve your item off the body.
Return to The Apprentice. He tells you why this elemental behaved differently.
This quest is part of the Conjuror Epic Weapon Timeline, and can only be started/completed by a member of that class of at least Level 80.