Probably the Perfect Paradox shotgun from the Curse of Osiris campaign? Not sure.
I have the Perfect Paradox and most of the Curse of Osiris weapons. You don't need to touch the Crucible to get them. You just pick up the prophecies from Brother Vance and fill them with Concentrated Radiolarian Culture, Advanced Paradox Amplifiers and that sort of thing. There are no quest steps involving doing any particular activities.