The husk of the pit is only available on a rare drop from a "sword of crota" which are no longer around after the HoW update. One story mission on TDB lets you kill one "sword of crota", but I am concerned that the gun "husk of the pit" is no longer available for a random drop now that they updated the game to the H.O.W. ?
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Fist of Crota, earth. But, I have not seen it drop for a couple of weeks now, when, it use to drop for me at least once a week per guardian toon. FWIW...