BG3 is based on the DnD 5e system, with some homebrew rules. Items have different tiers of rarity. While there may only be one in existence in this game, it’s not a unique item in Torril at large in the context of the Forgotten Realms.
I’m not saying that your idea is wrong. It makes sense in the context of this specific game. I’m just pointing out why it is how it is right now.
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