When it comes to Horcruxes, there are some powers you could gain by creating one, not only for yourself but also for the Horcruxes. However, many side effects come with this newfound power. It's essential to view the whole picture of what one will look forward to once they create a Horcrux, not only the benefits one seeks from it. Do the rewards outweigh the risks and side effects enough to be worth going forward with? That's up to each dark witch or wizard to decide for themselves, as many wouldn't dare even consider it, let alone go through with it.
Immortality is the power to be gained from creating a Horcrux. As mentioned in the previous lesson, if an intact Horcrux still exists, one will achieve immortality by continuing to remain in the world of the living, even if it's as a non-corporeal shade form if a deadly blow occurs to the witch or wizard, which is said that few would prefer over death. Luckily, if this were to happen, some known ways exist to regain a physical body. Firstly, the soul fragment can possess others to regain physical form, though it significantly shortens the victim's lifespan. The second known way is only achievable through the help of a second party, which is for them to perform rituals with spells and create or obtain a potion that can make a body for the soul fragment. Depending on the potion, it could create a body with severe limitations, known as a rudimentary body, often needing to drink the potion every few hours to keep the little health gained from finding a body for the disembodied soul fragment, with the main ingredients that have been used in it being unicorn blood, which the drinking of is said to lead to a cursed life, and snake venom. It's also possible to restore the true body using a regeneration potion, with its ingredients including bone taken from the father, flesh of a servant, and blood from an enemy. The Elixir of Life, known to extend the drink's lifespan if consumed routinely, could also be used to restore a true body, though this potion is procured from the Philosopher's Stone. The reward of immortality and the risk of losing one's true body is enough to encourage whoever makes a Horcrux to hide it away, keeping it safe while keeping its powers dormant.
The soul fragments embedded in each Horcrux can think for themselves, which means they can have particular magical abilities. Powers that come from Horcruxes are far more complex, as they can have general powers that may work positively or negatively, depending on who is around them, for how long, and more factors. A Horcrux's general power is the ability to mentally affect those around them, particularly negatively, such as by darkening their thoughts. This can be seen in Hermione Granger, Ron Weasley, and Harry Potter while they had the locket Horcrux, as it made them angry and moody, more prone to fighting, darkening their thoughts to the point none of them could cast their Patronus. The very same locket Horcrux, when worn by someone who has a more malicious deposition and natural ability in the Dark Arts like Dolores Umbridge, had a positive effect, gaining strength from the darkening aspect of the Horcrux and allowing her to still cast her Patronus. The longer someone is around the object, the worse the effect is, as the Horcrux is gaining power, though it's not usually a permanent effect. Time away from the object will wane its influence until the person is back to their usual self.
Along with getting stronger, the longer a Horcrux can be around and negatively affect someone, the more it can feed on one's lifeforce or even control them, as seen with Ginny Weasley while she had T.M. Riddle's diary. Ginny was emotionally vulnerable, often writing her fears and insecurities into the diary, which meant she spent a lot of time with the Horcrux and made her the perfect victim to control and drain the lifeforce from once the Horcrux had gained enough power. It had drained enough lifeforce from Ginny to physically manifest its soul fragment into a semi-corporeal form, leaving it capable of even using magic with Harry Potter's wand. Horcruxes have self-defense mechanisms, acting out when they sense danger to themselves. An example of the mechanism in work is with the Slytherin's locket, showing visions of Ron Weasley's worst fears to him and strangling Harry Potter when they had gotten close to destroying the locket. If a Horcrux hadn't accrued enough power, it might be unable to use this mechanism since Horcruxes that hadn't been near anyone in a long time were able to be destroyed without many problems. The creator of the Horcrux may also enchant it to have defensive properties to prevent destruction, like Marvolo Gaunt's ring containing a deadly curse for those who touch it, the Slytherin's locket being unbreakable even with the help of House-Elf magic, and T.M. Riddle's diary being waterproof and therefore unaffected by spilled ink.
Some principles apply to the nature of magic, with Horcruxes invoking the First Law of Fundamental Law of Magic made by Adalbert Waffling, stating, "Tamper with the deepest mysteries — the source of life, the essence of self — only if prepared for consequences of the most extreme and dangerous kind." The consequential side effects of Horcruxes are due to the tampering with one's soul and are not to be taken lightly. The first side effect of creating a Horcrux is dehumanization. There is no part left untouched by the dehumanization effect, as it's both physical and emotional, which becomes worse each time another Horcrux is made. The best showcase of the physical dehumanization would be in Lord Voldemort, who once appeared as any other wizard, who came to have an almost burned and blurred-looking appearance with unnaturally white skin, red eyes, and slit-like nostrils. The full extent of how much of his appearance was an effect of creating his Horcruxes is unknown since it's speculated that Voldemort underwent dangerous transformations as well, so his appearance is likely a combination of these factors. Another side effect is the instability of one's soul. Even after creating just one Horcrux, the main soul becomes unstable, which is the direct cause of Harry Potter becoming a pseudo-Horcrux as Voldemort's soul split too easily. The last known side effect is the inability to move on from Limbo. Harry Potter saw this in his trip to Limbo, where he saw the physical representation of Voldemort's soul as a mangled body, unable to move onto the afterlife from the corruption in his soul or return to the land of the living as a ghost. The figure was reported as "gasping raggedly for breath and crying in pain" when Harry saw it, appearing to be in constant agony. Soul fragments do not reconcile after death and have minimal awareness or intelligence due to the form they take in Limbo.