A Horcrux is a powerful dark magic object from the Harry Potter continuum. It is used to grant immortality by housing a piece of its owner's soul, ensuring that even if the wizard's body is damaged, some of their soul remains unharmed and bound to the world. A Horcrux may be created out of any ordinary physical object. The murder of another human is necessary to splinter the caster's soul and complete the Horcrux spell.
More information can be found on the Harry Potter Wiki.