What about the black thing that held Subaru's heart, could be a shadow form of the witch?
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The Black hands were the witch'es one.
We know it for sure when Beteleguse is first known for being in a cult that praise the witch and then at the episode where the witch killed Emilia and he is using them in attempt to turn Emilia's body apart he is saying "the hands that were granted by love" so it means he didn't have it before so its not his.