"A Horcrux is an object in which a person has concealed part of their soul.

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One splits ones soul and hides part of it... By doing so, you are protected, should you be attacked and your body destroyed.

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That part of your soul that is hidden lives on. In other words, you cannot die.

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Very dark stuff. Very dark indeed."

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-Horace Slughorn (Harry Potter and the Half-Blood Prince)