Horcruxes are dark magical objects created by splitting one's soul and hiding a fragment within an item. This process grants the creator a form of immortality, as they cannot truly die as long as the Horcrux exists. The act of creating a Horcrux is considered one of the darkest forms of magic, often involving murder to achieve the necessary soul fragment. In the Harry Potter series, the main antagonist, Lord Voldemort, created multiple Horcruxes to secure his immortality. Each Horcrux is tied to a specific object, such as Salazar Slytherin's locket or Helga Hufflepuff's cup, making them crucial to the plot as the protagonists seek to destroy them to defeat Voldemort.