Disclaimer: although the theories and information I present here is as internally consistent and as canon-consistent as I can manage, it is not verified, confirmed, dispensed, or in any other way affiliated with J. K. Rowling's own explanations. The goal of this is to add my headcanon to that of others, so don't mistake this for canon truth.
Harry Potter and all affiliated characters belong solely to J. K. Rowling.
The following files were compiled by Senior Unspeakable [REDACTED]
On immortality:
-Due to the Principal of Magical Balance, creating immortality requires ending a life. There are several different methods of immortality, with varying degrees of difficulty, effectiveness, and gruesomeness, but all require the death of at least one person.
-On Horcruxes
-A Horcrux is one of the simplest ways to become effectively immortal. All it requires is to cause the death of a sentient, sapient, unwilling creature, and to cast the ritual spell[1] at your target Horcrux. This causes your soul, stretched by causing death, to attach to the Horcrux as well as to your body. Superstitions about permanently severing part of your soul are all nonsense: a severed portion of your soul results in a new creature entirely. When a child is conceived, their soul is a mixture of pieces of their parents' souls. Horcruxes merely anchor the soul to the world in multiple places, so that the destruction of the body won't kill the person. Note that the person would become a disembodied soul, unable to perform magic or physically touch anyone. The soul can speak or possess a willing host or an animal.
-Most people who've made Horcruxes[2] only made one. There are five people who created more than one Horcrux, of whom none now survive: Gregori Rasputin, Abraxes[3], Tom Riddle Jr.[4], Amon the Cadaverous[5], and Vlad Drakul[6]. Records survive now for the locations and natures of Drakul's and Riddle's Horcruxes: for Riddle's, read the seven-book biography of Harry Potter[7]. Drakul's Horcruxes were three matching rings, all carved from a single large ruby.
-a) One was able to connect to the mind of the wearer and let them speak to Drakul from any distance and vice versa, as well as letting him possess them from a distance. He gave this to his second wife, and after her death to one of his servants. When destroyed, it released Fiendfyre around itself.
-b) One he wore himself. When his body was killed, the ring burned through his finger, flew at his killer, and released Fiendfyre, slaying him as well.
-c) The third was incased in a three foot sphere of iron, which was kept moving around his kingdom in a wagon under the Fidelius charm. The charm eventually degraded and it was found and destroyed, resulting in Drakul's permanent death.
-On returning from death (having created a Horcrux)
-Due to the Principal of Magical Balance, creating a new body for a ghost or other disembodied soul or spirit requires divesting another person of a body, if not outright killing them. There are currently three ways known to the Department of Mysteries.
-The first way was used by Riddle to 'return from the dead' in 1995. A pregnant mother had the child ripped out of her after seven months, seven weeks, seven days, seven hours, seven minutes, and seven seconds, with the child being a seventh pregnancy and conceived at midnight on New Year's Eve: the ritual must therefore reach completion at 7:07:07 on September 19th. This results in a total of seven sevens, which makes the child have the magical strength of whoever possess it. The ritual which rips out the baby also divests it of its soul, leaving it ripe for that possession. Riddle possessed it and was kept alive by means of snake venom and willingly given blood. The potion in the Graveyard, seen by Potter in 1995, was mostly aging potion, plus a donation of blood, bone, and flesh to fix the body in place, rather than shrinking down after it wore off. That's why the new body was so snakelike: because the body was of a possessed child raised on snake venom and almost nothing else. Using this method meant that Riddle's magical skills were equal to his original powers.
-Long-term possession of a willing subject, such as Quirrell in 1991 and 1992. Over about three years the subject would entirely lose control over their body to the possessor, and their body increasingly has the possessor's sticking out of the back. Quirrell only had Riddle for a year, and the head was already fully formed. Over the three years, the rest of the body would appear, and the original would fade into the back. Riddle could easily have used this route with Pettigrew. If Riddle had taken this route, it would have left him with a body just like his original one, but with only the magical power of the possessee; he would also have access to the knowledge and skills of the original person[8], and their advice if needed.
-A third and similarly unpleasant route would be to collect blood from a living relative[9], some bone from the person attempting to return, and an infusion of magic from a willing donator. This would let him craft a new body from the blood around his bone, and use the donated magic to draw his own strength back into the world. The person who donated some of their magic would lose some magical strength permanently, however. This would have left him with a body just like his own, but with strength equal to the magic donator before the donation. Note, however, that if the donator donates all of their magic, then the person returning would have their own magical power plus that of the donator. This route was also available to Riddle, as his mother's brother Morfin Gaunt was still alive, and the house as Godric's Hollow was left untouched, including his body. Pettigrew would have willingly granted his magic in return for his life. Due to the general superiority of this method to the one used (secrecy, simplicity, increased magical power, etc), it is believed that Riddle did not know of this method of returning to life.
-On other methods of immortality
-The Philosopher's Stone seems on first inspection to be a safe way to become immortal which doesn't require death. However, its color is known to be blood red. Its creation, while the exact process is not known, involves having a willing subject consume potions of longevity and potions of immunity to diseases, then participating in a ritual which drains their blood into a tub and crystallizes it into the Stone.
-The Stone, also, only lasts for enough Elixir of Eternal Life to make someone immortal for about 5000 years, since small parts of it are chipped off into the Elixir. Flamel was sharing it with his wife, so they would've lasted for about 2500 years; into the late 3000's.
-Although the Stone crystallizes as a perfect sphere, the chipping results in the miscellaneous rock shape it had at the time of its destruction. The Philosopher's Stone only prevents natural death by making the person ageless and immune to disease, not against violent death. Although Director [REDACTED] makes use of a Philosopher's Stone and inscribed the recipe for the Elixir of Eternal Life in our records herself, she has never divulged the ritual for creating a Stone, saying only that the price was too high.
-A third way is to use a modified time turner to Turn back your body at a constant rate of one hour every hour. This results in agelessness and effective immortality, and makes long-lasting curses and injuries heal almost instantly. If struck by the Killing curse, your body, being Turned back all the time, will keep living; but in the same way as if you were Kissed by a Dementor, making you a vegetable.
-Since a Horcrux won't keep you from aging[10] and this won't keep you from being killed by AK, combining a Horcrux and a Turner is considered by the Department to be one of the best methods of immortality[11]; you'll be immune to aging and disease, any curse other than the Unforgivables has basically no effect, and even AK won't work properly, since your now soulless body would be the perfect receptacle for possession. The only way to kill you would be to destroy both the Turner and your Horcrux(es)s.
-Binding the turner to yourself is simple and just requires a tiny piece of your soul and a drop of your blood in order to let it work on you and no-one else, and to let yourself use it from anywhere without turning it. The problem, and what requires death, is that it will still work like a normal time turner. To reshape it so that it'll work at a constant rate rather than simply move a person backwards, you have to sacrifice a (not necessarily willing) man who has recently traveled by that same time turner, and apply the blood of whoever made the time turner being reshaped.
[1] The spell takes approximately five minutes and a small blood sacrifice from the caster.
[2] 387 are known to the department, 369 of whom are currently deceased.
[3] Abraxes posed as the god Hades in ancient muggle Greece.
[4] Riddle is better known as Lord Voldemort, You-Know-Who, or He-Who-Must-Not-Be-Named.
[5] Amon posed as the god Osiris in ancient muggle Egypt.
[6] Drakul is also known as Count Dracula.
[7] The volumes are also available in cheaper, non-charmed form in the muggle world, as fiction.
[8] For example, if the possessee was an animagus, the possessor would keep the ability to transform into that form.
[9] The ritual requires all of their blood, so the living relative would die.
[10] Although you won't age to death, you will age to near-uselessness
[11] This was the method used by Herpo to Foul for some 250 years, keeping him at the physical age of thirty.
A/N: As I hope you all are clever enough to figure out, this fanfiction is written as files from the records of the Department of Mysteries, and thus is technically classified. Don't tell anyone you read this: shh! Anyway, I do have three other chapters written up already, but if you want more information on a subject just leave a review asking. Criticism is welcome, flames are not.
Canon violations in this chapter: It would be very difficult to acquire a fetus body to properly fit the ritual, especially given that Voldemort apparently had the fetus body at the beginning of GoF. The soul is severed to create a Horcrux, rather than merely stretched, as here.