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I have killed like over 50 world bosses, checked every trader, open many treaure chests, may have killed over 100 delve bosses but have not found Plague Doctor legs.
Got every piece of set except this, i am on PS4 NA.
Pc may have higher rate but facing issue on consoles..
They are pretty rare when compared to sabatons, helmet and cuirass. I don't know why the drop chance on the greaves is so low, but at least you know they're there