"Encase? But how — ?"
"There is a spell, do not ask me, I don't know!...Do I look as though I have tried it — do I look like a killer?"
-- Tom Riddle and Slughorn discussing how to make a Horcrux (HBP23)
A series of dark spells plus murder are among the steps to complete a Horcrux (PC/JKR1).
References from the canon
- Horace Slughorn told young Tom Riddle that evil spells were used in making a Horcrux to encase part of the soul, but he had no idea what they were (HBP23). Riddle discovered that information in a book at Hogwarts called Secrets of the Darkest Art, which warned that "strong enchantments" must be put on a Horcrux to make it virtually indestructible (DH6).
- Snape told Dumbledore that the curse used on Marvolo Gaunt's ring was the type that would "strengthen over time" and could only be contained for a year before killing him (DH33)
- Making Horcruxes is a "grotesque" process. First a wizard rips his own soul through a vile act of murder on an unsuspecting victim. Next, a "receptacle" object is cursed with unknown dark spells in order to encase the soul piece. Third, other acts are performed that are nearly unspeakable (PC/JKR1).
- Harry was never a true Horcrux because he did not have the curses (plural) that other objects had placed on them, and thus was not magically enchanted to accept part of Voldemort's soul. Harry's soul remained intact and he was "in essence divided" from the soul-fragment under his scar (PC/JKR1).
In an interview in 2007 with Pottercast, J.K. Rowling hinted that other rituals connected to the formation of a Horcrux were too disgusting to explain to readers:
I see it as a series of things you would have to do. So you would have to perform a spell. But you would also-- I don't even know if I want to say it out loud, I know that sounds funny. But I did really think it through. There are two things that I think are too horrible, actually, to go into detail about. One of them is how Pettigrew brought Voldemort back into a rudimentary body. 'Cause I told my editor what I thought happened there, and she looked as though she was gonna vomit. And then-- and the other thing is, how you make a Horcrux. And I don't even like-- I don't know...(PC/JKR1)