From what I have seen, husk of the pit will drop from any hive activity.
Step 1, kill approximately 100 knights.
Step 2, visit the speaker then Eris morn
Step 3, complete The Abyss, kill crota on 390, kill omnigul.
That's as far as I have got? Anyone got any further?
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I've only heard of husk dropping from non-yellow bar ogres. It can drop requiring either thrall kills, knight kills or wizard kills.