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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The mission you have to do for the quest was pretty fun by myself. It answered the question of "How many Knights can we chase you with while also denying you 80% of the floor?"