A Horcrux is a very powerful object in which a Dark wizard or witch has hidden a fragment of his or her soul for the purpose of attaining immortality. Creating one Horcrux gave one the ability to anchor one's soul to earth if the body was destroyed. The more Horcruxes one created, the closer one was to true immortality. Creating multiple Horcruxes was suggested to be costly to by the creator, by both decreasing their humanity and even physically disfiguring them.
The first Horcrux was created by Herpo the Foul. The only other known creator of them was Tom Marvolo Riddle, who was possibly the only one to have successfully created more than one Horcrux. In fact, Voldemort created seven Horcruxes. Horace Slughorn mentioned that the fate of those who used Horcruxes was that only few would prefer, which suggested that few others had created their own Horcrux.

The nature and concepts of Horcruxes were so terrifying, they were kept secret from most of the wizarding world and only few ever knew what they were. As little was known of the subject, no one knew what the effects of creating multiple Horcruxes would be, because, aside from Tom Riddle, nobody had done so.