Keep in mind that Charlemagne sent a nameless knight to rescue Joyeuse. That knight will likely represent the playable character/guardians.
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So maybe if the unnamed man in the white mask is Charlemagne's parallel and he used to rule over Mars after the traveller arrived, then was conquered by the Cabal (his greatest defeat). Now he is a prominent figure in the City, and will be sending us guardians to retrieve Joyeuse from his treasury. But did the nameless knight ever return with Joyeuse? And if so what did Charlemagne do with it once he had it again?
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If Joyeuse was an AI or weapon ten it would make sense that we would all be sent to retrieve it and be able to use it since it is "Anyone's Sword"
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A nameless knight, that's -blam!-in' clever, Bungie! Good catch.