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It isn’t precisely clear what “Dark magic” is, so here are a few common theories.
“Dark magic” is a catch-all political term for “bad magic.” If the people in power don’t like it, if the spell is taboo, then it’s Dark, simple as that. Less cynically, “Dark magic” is a category that covers one or more of the items below but, as any surgeon would advise, the nature of life in this world requires a more sophisticated view than “spell that only hurts people = bad.” Jinxes and even many hexes are technically Dark magic but they are unrestricted nonetheless because their harm is minor or sometimes useful (e.g. many parents want to use the Birching Jinx on their children).
Dark magic is dangerous. Normal spells do what you tell them to do, and you can count on them working exactly as intended if you do them properly, but Dark spells are like…not an evil genie, necessarily, but an ill-mannered one at least. The spell may not work as planned if your conscious intent and your deeply-held desires are not in alignment, or if you do not have the willpower to control your spell (e.g. Fiendfyre can fuck you up, not to mention everyone and everything that had to misfortune to be next to you).
Dark magic is permanent. Whether you’re sacrificing a drop of blood in a Dark ritual or cursing somebody’s eye out of their head, the thing that you’re doing cannot be undone. There may be a possible exception in the use of other Dark magic, but if all Dark magic is harmful somewhere along the line (see below) then it’s easy to see how Dark magic makes the world just a little bit worse off because the damage that it inflicts can only be shifted elsewhere, not actually undone.
Dark magic’s sole purpose is to inflict harm. You can kill somebody with a Severing Charm or drop a levitated anvil on somebody’s head, but these aren’t Dark spells because there are legitimate uses for them, whereas e.g. the Bat-Bogey Hex is going to turn somebody’s bogeys into bats, which has pretty much zero non-harmful applications.
Dark magic requires a desire to harm. We know that Cruciatus Curse requires an intent to inflict pain, and other Dark spells may be similar. Perhaps the Killing Curse requires a desire for the target to be dead, full stop. The Killing Curse may require a desire for the target to be dead, full stop. If these desires must be ends in themselves then that may be why the Unforgivables are unforgivable: simply using them is evidence that there were no mitigating circumstances.
Dark magic harms the caster. Long-term use may physically degrade your body (e.g. Voldemort’s red eyes) or harm the mind. If Dark magic is more about will than words, then some of the harm may be an increasing tendency to use Dark spells (in general, or just those that are frequently used) without really meaning it, blurring the line between “the thoughts that come into your head” and “the thoughts that you choose to act upon.” If Dark spells requires certain desires, then their use may encourage those desires, e.g. wixes who frequently cast the Cruciatus will become more sadistic (which will likely encourage more regular use of the Cruciatus, and so on and so forth).
Dark magic is inextricably harmful somewhere along the line. Where Dark magic is concerned, there is no such thing as a “victimless” spell: even if the object of a Dark spell is not to inflict harm, someone will nevertheless be harmed, e.g. Voldemort’s resurrection ritual, which was possibly Dark on the basis of stealing Tom Sr.’s bones (if disrespect for the dead counts as harm), probably Dark on the basis of Pettigrew’s donation (if voluntary sacrifices still count as harm), and definitely Dark on the basis of Harry’s blood sacrifice.
Dark magic is any magic that is shaped more by will than by word. The ideal spell is like the Levitation Charm: you wave your wand and you say the right words, and if your intent is clear then the thing happens, end of story. Dark magic, on the other hand, requires stuff like “having a genuine intent to cause pain” or “holding a happy memory in your mind” (yes, the Patronus Charm is technically Dark magic, but, well, see that first item about the term being a political one). It may also be easier to cast nonverbally and/or wandlessly, and if long-term Dark magic use harms the caster then the “harm” may be an increasing tendency to use Dark magic without even really intending to do so.
The upside to all that wand-waving and incantation-speaking is that it’s hard to accidentally blow up your aunt, at least once you’ve gotten your magic under control (and yes, accidental magic is also, technically, Dark magic — the earliest “spellcasting” may well have been attempts to intentionally trigger accidental magic to achieve specific ends). This connection between Dark magic and accidental magic is suggested when Snape says — with the obvious expectation that he will be believed — that he thought that Harry survived Voldemort’s Killing Curse because “he himself was a great Dark wizard.”
Dark magic is unorthodox, primitive, outside the bounds of acceptable practices, etc. This brings us back to the first item and doubles down on it: “Dark magic” is, first and foremost, not the way that things ought to be done these days. There are norms to magic as it is generally taught and practiced in Britain, and Dark magic is just the shit that falls outside these norms. The British system of magic is a system, limited yet reliable, and Dark magic breaks those limitations by operating outside of them. What is Dark in modern Britain might not be Dark in Medieval Britain or modern Scandinavia, if magic were systematized differently then or there.
On Reddit, u/Uncommonality suggested that “Dark magic” in Britain is rooted in traditional magical practices that were suppressed following the Norman Conquest, plus the sort of monstrosities that inevitably crop up when you start a Back To Basics Club but your only entrance criterion is, “Do you want to do some stuff that ordinary society doesn’t like?” If A.D. 1066 marks the division between the Dark Arts and modern, acceptable magic, it’s worth noting that Christianity isn’t necessarily verboten to Dark magicians, although if modern practitioners are more like historical reenactors and Romaboos than Hidden Christians and Crypto-Jews, there isn’t necessarily an actual chain of practitioners to connect them to the past and they may very well resurrect pagan practices out of ignorance or preference.Grindylows, nogtails, and their ilk are “demons,” but neither the core seven books nor any other source has ever explained what that means.
One approach: Outside of a Christian context, demon usually means “destructive, malicious, generally evil creature or spirit” or (more often as daemon) “tutelary or guiding spirit.” I think that we can ignore the latter, which means that grindylows and pogrebins are probably akin to utukku, shedim, and so forth: they are destructive beings that are inimical to humankind, perhaps even actively malicious, in a way that is unlike other beasts — the wolf and dragon prey upon sheep and men, and the Chizpurfle and rat infest wands and larders, but all this is in the furtherance of their own life cycles.
Even the ichneumon wasp seeks merely to propagate itself when it paralyzes and parasitizes a caterpillar, but the grindylow will throttle you for the joy of throttling and the kelpie will slay by drowning despite the sharpness of its teeth. They delight in misery and suffering, and their urge for it is no less deeply-rooted than another animal’s need for enrichment; grindylows fed upon a diet of dead fish will not waste away, but they will grow listless and may resort to self-mutilation just to inflict pain on something. This drive may even be their most powerful: nogtails will eat and mate as the opportunity arises, but one who is deprived of both food and suffering will sooner choose to inflict misery than to eat, perhaps even if it were on the edge of death.
Another: Demon is simply another word for “Dark creature,” in which case the previous section may be of some use. Perhaps wounds inflicted by a demon cannot be healed by most magical methods. We know this is true of werewolves even in their human form (which may go far in explaining why many wixes consider werewolves to be dangerous every day of the month).
A third: Demons are more fundamentally rooted to magic than most creatures. In Taure’s canon, House-elves and veelas can reproduce by natural means but may also generate spontaneously under the right conditions. Demons are similar, but among “the right conditions” is the use of Dark magic.
Thanks be to Cog and Star, who put together the original version of this list.
It is only possible to swear one Unbreakable Vow, either “at all” or at least “at one time.” This would probably be the case for all three parties.
Unbreakable Vows are extremely subject to unintentional consequences. If you accidentally break the Vow, you die. If the Vow requires that you do something that subsequently becomes impossible to do, you die. If you fail by a fraction of a centimeter, you die. Ambiguous language in the Unbreakable Vow often makes death more likely, not less.
It is not possible to release the Vower from their commitment. Even if circumstances change or the Vowee changes their mind, the Unbreakable Vow remains unbreakable.
Each party to the Unbreakable Vow must be participating fully freely. Any degree of coercion, even as benign as “You will not be given this job unless you swear an Unbreakable Vow to follow its associated code of conduct,” will prevent the ritual from having an effect.
One or more parties must already be trusted by the other. For example, the Vowee must already trust the Vower will carry out the terms of the Unbreakable Vow. In this case, the Unbreakable Vow serves a purpose other than ensuring that the terms are followed: it may be a sign of dedication meant for the benefit of others (just as Snape proves his earnestness more to Bellatrix than to Narcissa) or as a kind of terminal backup plan: swearing an Unbreakable Vow to kill someone on a particular night means that, if you fail and are captured, you will swiftly die and deny your captors the opportunity to interrogate you.
The Unbreakable Vow affects the soul, and is therefore incompatible with many other forms of soul magic. For example, if there is an Unbreakable Vow wrapped around your soul (so to speak) then you cannot be the Secret Keeper for a Fidelius Charm, which requires that the “secret” be placed in your soul. This could interact with Horcruxes in multiple ways: Voldemort might simply be unable to swear an Unbreakable Vow, or (more interestingly, in “yes-and” fashion) each soul-fragment of a Horcrux could be a separate party to an Unbreakable Vow. Also, maybe violating an Unbreakable Vow doesn’t just kill you but destroys your soul.