All you get are legendary gloves, NOT the precursor to necrochasm.
Just a PSA so you don't get your hopes up.
EDIT: The precursor to necrochasm is Husk of the Pit, it DOES NOT drop from the hard raid (hotp). HOTP is supposedly a quest reward and can be EVOLVED into necrochasm.
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I only did it for rep. Actually I mostly did it to take the damn quest marker away from the tower.