From "WikiDex: The Free Encyclopædia"
Retrieved September 22, 2013
Dark Magic: Horcrux
One of the darkest know magical artifacts is known as a horcrux. While the exact methods of constructing a horcrux are kept secret, much like with other kinds of dark magic, it is known that a integral part of their construction, and likely the last step of the ritual, is murder. The victim doesn't have to be magical, though it is thought by many magical researchers (such as the British Ministry of Magic's Department of Mysteries) that a magical victim enhances the properties of the horcrux.
A horcrux, when created, contains a half of the creator's soul, including half of their magical power. Many dark mages will complete power-enhancing rituals either before or after the creation of a horcrux to mitigate this loss of power.
The main utility of a horcrux is that is ties a portion of the creators soul to the object, preventing the rest of the creator's soul for dispersing after death (or forming a ghost). This will allow the creator, if killed, to return to life, either by possessing a mage (though only for short periods), or via various resurrection rituals (which only function for those with horcruxes, and are thus quite rare, esoteric, and/or complex), most of which were invented by various dark mages independently to return to life after death.
The largest side effect, aside from loss of magical power, of creating horcrux is the detrimental effects to the creator's psyche. Known psychological effects include, but are not limited to, megalomania, multiple personalities, loss of empathy, and loss of sexual desire.