The basic definition of a Horcrux is "a vessel that contains a portion of its creator's soul". But, like so many other magical objects and rituals, Horcruxes are more complicated than their basic definitions imply.

For instance, there is the oft-disputed function of the killing curse. In the Horcrux ritual, the killing curse serves two functions: it makes the caster's soul slightly unstable and provides energy for the chain reaction of spells that follows.

By the way, the act of murder does not actually split or tear the soul. It merely creates a sort of hairline crack that makes it easier to actually divide the soul later.

The death serves as a sacrifice for the ritual. In ancient times, wizards would often sacrifice someone to aid in particularly powerful magic, and it is still occasionally done for certain magical rituals today. Basically, the sacrifice releases the victim's life-energy (also called qi, prana, etc. by various cultures). If captured, this energy can fuel some very intense spellwork. In the Horcrux ritual, a modified version of the killing curse carries out this function.

After the sacrifice comes the actual division of the soul. This is the most difficult part of the ritual, and can lead to the Horcrux-maker's death if cast without enough energy or intention. I will give the actual incantations in the next chapter of this book.

Dividing the soul is extremely painful, and shouldn't be attempted by the faint of heart. There is no way to really describe this pain, except that it is both physical and mental as well as very severe. It causes many witches and wizards to lose their concentration and accidentally end the ritual during this step.

Usually, the soul is cut roughly in half. The experienced practitioner making a second Horcrux can visualize this step and concentrate on a smaller piece being removed, but they must be very careful in making sure they end up with the larger half in their own body.

Once the caster's soul is divided, they must say a two-part spell that makes the torn portion come forth to the tip of the wand and then escape into whatever object the wand first touches. This step is relatively simple, unless the wand is touched to an unintended object.

Finally, the piece of soul must be permanently sealed into its vessel. The short spell that accomplishes this step also gives certain properties to the vessel itself, making it resistant to most methods of destruction. (The known exceptions to this rule are Fiendfyre, basilisk venom, and the Sword of Gryffindor.)