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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When you play the game for the rewards instead of just enjoying the game you will always be disappointed. Why do you really care what you get or what it does or does not lead to? We get more content, a longer bounty.. and it's still never enough. I don't get it.