It was Voldemort´s soul, or what was left of it. That´s why voldemort has to wake up as Harry in the forest, and that´s why Dumbledore tells Harry: he is more afraid to return here than you are. Also Harry at the end says that Voldemort should try for some remorse, for Harry saw what he would be otherwise, just what was left of his soul after the Horcruxes.
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