In the chapter `` King 's Cross '' , Dumbledore 's spirit tells Harry that when Voldemort failed to kill baby Harry and disembodied himself , Harry became an unintentional Horcrux ; Voldemort could not kill Harry while the Dark Lord 's soul shard was within Harry 's body . Dumbledore informs Harry that when Voldemort disembodied himself during his failed attempt to kill baby Harry , Harry became an unintentional Horcrux ; Harry could not kill Voldemort while the Dark Lord 's soul shard remained within Harry 's body .