Moonfang armor can be dropped when defeating the Kell Echo (not from the end chest, the reward immediately after killing the Echo)
Since Armor doesn't drop from any other source in Rite of the Nine, I must assume this is unintentional.
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iirc the moonfang armor drops specifically from killing Kell Echo himself its why you get it immediately as opposed to the chest even in normal prophecy. Its still there since it doesn't take away from the chest drops anyway.