If you have a Rare Ergo in Lies of P, there are a few things you can do with it. Here are some options:
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Hold onto it until you find Alidoro in Area IV, St. Frangelico Cathedral. You can exchange Rare Ergo for Special Weapons and Amulets.
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Exchange it for some insane boss weapons and amulets by following the instructions in this video.
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Wait for the full game to come out, as there may be a special merchant for it like there were for Boss Souls in Dark Souls.
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Exchange it for a unique weapon and amulet by following the instructions in this guide.
Ultimately, what you do with your Rare Ergo depends on your playstyle and preferences.