In canon we have three people who knew about horcruxes before the reveal: Dumbledore, Slughorn, Voldemort. Presumably, all of them learnt about the topic from the Hogwarts library. After that, we have Harry, Hermione and Ron. That's it.

In fanfic however, we see a whole range of people who know. Some authors seem to treat them like common knowledge for anyone who knows something about the dark arts (which is ridiculous unless some event causes it to happen, such as the Prophet reporting on the secret of Voldemort's immortality), and others various shades of prevalence.

I'd like to say the following people probably knew about horcruxes:

• Grindelwald - Dumbledore level of talent and knowledge, probably consumed every book on magic and dark magic he could get his hands on.
• Some of the Unspeakables researching souls in the Department of Mysteries.

These people might have known:

• Snape - Liked to research all about the dark arts, similar skill level to Slughorn but more interested in the topic, knew enough about dark magic to treat Dumbledore's obscurely cursed arm. He probably had read about them before deciding anyone who tries it is a fool.
  
• Some goblins - A few goblins had probably come across a book on the topic at some point, or if Gringotts raided more than just Egyptian tombs, they may occasionally come across them. They should be as obscure among goblins as they are around wizards though, so no counter goblins or minecraft goblins knowing about them.
  
• Rookwood - Must have been pretty talented to end up as an Unspeakable. Probably liked researching dark magic like Snape.

These people may have heard of them in passing or heard of the concept:

• Bill Weasley - Most Egypt stuff predates horcruxes, but if Gringotts works elsewhere, he might have heard about the concept in passing from another curse breaker.
• Moody - if they are a roughly "once a century" thing in the UK, he might have heard of them or encountered one over his long career. This strongly depends on how common they are, but I could buy them occurring that frequently.
  
• Sirius - If there was anyone who had a copy of Secrets of the Darkest Art, it would be the Blacks. I can buy Sirius having heard about the concept at some point, but I would not buy him knowing much more than that. He would have been revolted before putting the book back on the shelf.

After these people, anyone who knows either does because they encountered one and went looking to find out about it, or stumbled onto the topic by sheer luck.

Thoughts? Anyone I missed?

 

This all has to depend on the restricted section, and how actually restricted it is, right? I'm not asking for wizarding spelunking through book caves with lifelines like in Taure's VP, but... Secrets of the Darkest Art, the book that gave instructions on a horcrux's creation and destruction, would have allured any budding dark wizard, or shit, even anyone with the slightest curiosity on the dark arts. (Prior to APWBD removing it from the library in the 50s) It's apparently a medieval text, so the language barrier between english (or medieval greek? maybe? herpo the foul and all that) and archaic english would exist, but it probably wouldn't be much of a barrier to someone used to deciphering and translating literal runic text. I could see the text being unique, actually. Handwritten and charmed.

'Ol Tom only went to Slughorn because he was a psychopathic shit that disregarded the whole murder thing and wondered about creating more than one.

The knowledge is abhorrent, but useful. Useful with many sacrifices re. half lives and shit, but useful. Anyone finding the text and understanding it with the desire to use it would use it. Horcruxes would be everywhere. There has to be a limit on the access to the text somewhere. If any half ass malicious student could get the text with their restricted section pass?

 

They give out free pamphlets to anyone who wants to know in Knockturn Alley /s

On a more serious note, did Regulus know? Or did he just know that he'd found something dark, dangerous, and important? If you say Sirius could know because the Black library - which I can agree with - then Regulus certainly could, and you could also include Bellatrix and Narcissa (maybe not Andromeda).

 

Oof, not sure how I forgot that one. But yeah, Regulus is a pretty obvious one and him knowing puts my estimate of Sirius knowing up a notch.

Did the sisters also live at Grimmauld Place? My impression was that they lived somewhere else. We heard of Sirius' and Regulus' rooms, but not those three.

 

The Blacks definitely had something on the subject. You forgot to cite Regulus among the "in-the-know" people. I doubt Voldemort had told him, and even if he mentioned something in front of Kreacher, it wasn't explicit enough that Kreacher himself knew what the locket was, so I think Regulus had to have some previous knowledge to guess what the locket was.
Bellatrix also was very worried about the Cup, but that might just be her natural fanaticism.

On the other hand, the fact Lucius never suspected the Diary's true nature (according to Dumbledore, at least) would kinda hint that most people don't know, even amongst those interested in Dark Magic. That points to it being restricted to very few books, and also very rarely used. If every other dark wizard used them, you'd expect more people to know.

Outside the Dark-Arts oriented people, I guess some history maniacs would know of their existence, if only because they would probably feature in their makers' biographies. Though maybe they'd only know of them as semi-legendary, like Binns thought of the Chamber.

 

Re. Bellatrix. The Dark Lord had given her an artifact and told her to treasure it and protect it beyond all else, even (especially her life? but that's all supposition). I think that's just the fanaticism you describe. She's also after the time-frame from when Dumbledore removed the text from the restricted section. You could argue that the Black Family would have access to it, but I think it fits better that the text is almost unique. Maybe it has been transcribed, maybe it has bootleg versions.

Given that last, I can accept that she and the rest of her family would have passing familiarity with the term, but no knowledge as to what it really is other than, "super dark. much cool. might live forever". The Dark Lord isn't the type to share the secrets of his immortality beyond, "here's this thing. protect it. or else."

The message R.A.B. left throws some forks into that, but meh. Maybe he was familiar with the term and followed its secrets to the font of true knowledge. But Bellatrix seems the type to take it on face value? Maybe? It all ties back to how accessible the text of Secret of the Darkest Art is, and how accessible the derivatives of that text are.

The technique itself is too useful for anyone that lacks the scruples against murder to ignore. It can't be too common.

Otherwise every dark wizard would have one... and maybe they do? There has to be a limiting factor somewhere, and skill cannot be it. If any fuck off can get the knowledge and has the shithead mental disorders required to kill to live evermore, the limit has to be accessibility or ability.

If accessibility is the problem, there has to be a reason why considering it was freely there for any fucker that has a pass prior to the 50s.

 

What? I forget the relevant book passages, can you cite them?

 

If the HP wiki is right, it says in Deathly Hallows, Chapter 6 that Dumbledore removed Secrets of the Darkest Art from the library when he became headmaster. So it's a safe bet that they each learnt about the book and the topic in their Hogwarts days.

 

Here's Hermione, Ron and Harry speculating about it in Deathly Hallows:

 

Gotcha. The Hogwarts Library is pretty yikes.

 

Also would suggest that the Flamels might be familiar with them; since Nicolas was taking a different path towards extending his life, he may have researched other ways. It may even have been his insight that pointed Dumbledore to consider it when Voldemort had an unusual reaction to having his Killing Curse reflected back at him.

I'd like to think that the legend of Koschei the deathless was enough of a breadcrumb trail to guide Regulus into guessing what his master had been up to with Kreacher, hiding something in a remote cave with layers of protection. Whether the fanon Black Family Library of Awfulness actually covered it, I doubt Sirius would have found such a book unless it included woodcut pictures of sexy witches.