Okay then...on with the show!
Huge thanks to savingharry for compiling this summary for us!
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I would like to thank missmugglebethany, El Barto, and kyp for their assistance in writing this compilation. Also, I would like to thank hp6 and Albus Dumbledore for letting me quote them.
First, let's start with a note about what we know IS NOT a horcrux: the sorting hat.
JK Rowling has said so herself on her website:
QUOTE
Section: Rumours
(SPOILER WARNING)
The Sorting Hat is a Horcrux
No, it isn't. Horcruxes do not draw attention to themselves by singing songs in front of large audiences.
(SPOILER WARNING)
The Sorting Hat is a Horcrux
No, it isn't. Horcruxes do not draw attention to themselves by singing songs in front of large audiences.
I repeat: JK Rowling has said the sorting hat IS NOT a horcrux! So don't post saying that it is unless you believe Rowling is making such a statement to throw us off of the trail.
That being said, there are six horcruxes in total. Remember, the seventh piece of soul is within Voldemort himself.
Soul pieces we can identify:
1. Voldemort himself
2. Gaunt's Ring (destroyed)
3. Riddle's Diary (destroyed)
4. Slytherin's Locket (taken by RAB)
Dumbledore says that the three other pieces of soul are as follows:
5. Hufflepuff's Goblet
6. Nagini
7. Something of Ravenclaw's or Gryffindor's
This of course, remains to be confirmed. However, theories abound as to what the final three horcruxes could be.
Horcrux Suggestions
The following have been suggested as potential horcruxes. Arguments for and against are listed, and writers of the theories are noted. I have not included for or against on the Goblet, because general consensus is that it is a horcrux.
Gryffindor's Sword
For: by savingharry
Dumbledore has been known to have been wrong in the past, and even admits himself that he makes mistakes. We do not know where the sword came from before it came out of the sorting hat. It might have been in Dumbledore’s possession before that point, but we do not see it in the office on the first visit into the office. Only on the second, after the defeat of the basilisk in the CoS. So it could have been magiced from any place. Perhaps Voldemort gained access to wherever it was magiced from before the events of book 2.
Against: by missmugglebethany
There is a simple reason why Godric Gryffindor's sword can be ruled out. In Half Blood Prince, pg. 505 of the deluxe version American copy, Dumbledore says the following: "I am confident, however that the only known relic of Gryffindor remains safe.' Dumbledore pointed his blackened fingers to the wall behind him, where a ruby-encrusted sword reposed within a glass case."
In Order of the Phoenix, in the beginning when Lupin, Sirius, and the rest that were there giving the rundown on what the order was doing Lupin says, "Dumbledore's shrewd ideas normally turn out to be accurate." In Half Blood Prince, Lupin, Harry, and Mr. Weasley are talking. Lupin can be quoted saying the following: "People have said it, many times. It comes down to whether or not you trust Dumbledore’s judgment. I do."
Dumbledore firmly believed that the sword has been kept safe, and furthermore is not a horcrux. So we should trust Dumbledore and also believe that the sword is not a horcrux.
Nagini
For: by savingharry
The reasons for Nagini being a horcrux seem to be similar to those of missmugglebethany's reasons for Gryffindor not being one. Basically, Dumbledore said it was one. He's the greatest wizard of his age. He's worth trusting. Also, if Voldemort had found a sixth object to use when killing Harry, the object might have been destroyed or be unretreavable now, and Voldemort would be forced to use what was available.
Against: by savingharry
The reasons against it are also similar to the reasons for the sword. Dumbledore has been known to be wrong. Also, putting horcruxes in living things is tricky, and Voldemort might not have wanted to risk it. A further question would be, if Voldemort had found an object to turn into the final horcrux (when he was going to kill Harry the first time), why would he have not simply retrieved it, using it to make the final horcrux? Wouldn’t that be better than a live snake?
The tiara that Mrs. Weasley is letting Fleur use in the wedding
For: by savingharry
The tiara at the end of HBP being a horcrux comes up again every so often. A lot of the theory seems to be based upon misinformation, however. To understand any of this, it is first important to remember that Molly, despite being in Gryffindor, is a Prewett by birth, and thus might be a descendent of Ravenclaw. The first reasoning behind this is that the Prewetts are normally in Ravenclaw. I have actually not found anything anywhere that says this, other than forum posts. The origin of this claim might be that the Prewett family crest (in genealogy studies) has a raven on it, and that the family motto refers to a raven. The second reason behind the tiara being Ravenclaw's is that the Grey Lady, the Ravenclaw ghost, wears a tiara. However, we are not told this in the books at all. In the short references to the grey lady in the book, there is no mention of a tiara. Likewise, the Grey Lady in the films does not wear a tiara. The third reason behind this is that the tiara is goblin made. The argument is that, because Flitwick, the Ravenclaw head-of-house, is part goblin. This last part is true, and can be verified in the extras section of jkrowling.com. Related to this theory is that the tiara in the Room of Requirement in HBP is this tiara.
Against: by savingharry
The major problem with this theory is its lack of evidence. Most of the things used to make this claim are not verifiable fact. The only real verifiable fact is that of Flitwick's origin, and his relation to the tiara's origin seems to be very superficial. Ravenclaw lived hundreds of years before Flitwick was born, so his being in Ravenclaw's house seems to be of no relevance to his being part goblin, unless he is actually a descendent of Ravenclaw. Also, all good silver is goblin-made in the wizarding world. For example, the cups with the black family crest. Flitwick's being part-goblin and Ravenclaw head of hosue seems to be mere coincidence. As well, it does not seem to make sense for the Prewetts to be in possession of a horcrux. How would Voldemort have come into possession of it from the Prewetts? And how did it fall back into their hands again?
Hogwarts
For: by missmugglebethany
Draco dormiens nunquam titillandus
"A sleeping dragon [is] never to be tickled/poked." Those are the words inscribed on the school crest of Hogwarts. Hogwarts is the closest thing to love Voldemort ever know. Voldemort tried to put his dark influences in the school. Dumbledore denied him that. Putting his soul in something that Dumbledore and so many of his enemies treasured would be a huge trophy for him.
There are little hints throughout the book about Tom Riddle, his connection to Hogwarts, and how he would want items that are grand and with deep magical history. Dumbledore says that he believes that Voldemort may not have gotten something from Gryffindor because that the only known relic is known to be safe, so what would be better than taking the school as a whole? Salazar left the school because of his conflict with Godric concerning the admittance of muggle-born students. Salazar left mad. What better homage to the great Salazar than to take the school back. As long as it stands, true evil will prevail.
Some say that Dumbledore would know Hogwarts is a horcrux. Not necessarily. Dumbledore is not completely aware of evil lurking in the school. Remember, he didn’t know that Voldemort himself was in the castle attached to Quirell. As well, the diary was in the building, and that was a horcrux.
Against: by savingharry
There are several arguments against Hogwarts being a horcrux. The first argument is that the school might simply be too big to be a horcrux. Albus Dumbledore once asked that, if it were possible to make such a large item a horcrux, "Why not make the earth itself a horcrux for then you would never have to worry about it?" A second argument against Hogwarts being a horcrux is that the method for destroying a horcrux altering or destroying them. Either of these destruction methods might argue against Hogwarts as a horcrux. First, as hp6 says, if a horcrux is destroyed when altered, "then every time the staircases change then the horcrux is ruined, or a staff member is changed or a new plant is planted and so on." Secondly, if the school must be destroyed in order for the horcrux to be destroyed, then no one could go on to teach. Jo has already said that one of the students becomes a teacher at Hogwarts. Unless this happens before Voldemort is defeated, then Hogwarts will reopen and not be destroyed.
Horcrux Theories
The following are general theories about the horcruxes that come up. Things detracting from the theory are included when applicable. I apologize for the length of the "4 horcrux" theory. It is a fairly complicated subject, and so the topic was unusually long.
The locket in OotP was Slytherin's locket
By savingharry
Many people believe that Regulus Black is RAB. That would mean that Regulus was in possession of a horcrux, specifically Slytherin’s locket. In the Black House cleaning section of OotP, pg. 116, it says "...until Ginny had the sense to slam the lid shut; also a heavy locket that none of them could open, a number of ancient seals..." It has been theorized that this was actually slytherin's locket. If this was actually Slytherin's locket, then it could now be in one of several places. First, it could simply be in the trash or somewhere still in the house. Second, it could be in the possession of Kreacher, who "rescued" several items that were supposed to be thrown away. Third, it could be in possession of Mundungus, who stole several items from the Black house. Finally, it could be in the possession of Aberforth Dumbledore, Albus's brother and the barman at the Hog's Head. He was seen purchasing stolen Black family items from Mundungus in the street.
Horcruxes try to posses people
By savingharry
The theory behind horcrux possession has its roots in the only horcrux we have actually seen in action, the diary. Though the diary was, admittedly, unusual, the theory is that all horcruxes act similar to it, specifically, that it feeds off of the souls of others. The diary tried to bring Voldemort back to life out of the piece of soul in the book. The theory is that other horcruxes do the same. The ring, for example, was (according to this theory) attempting to feed off of Dumbledore’s soul, and that was why Dumbledore's arm was dead. Dumbledore had to kill his hand to destroy the part of the soul that had attempted to feed off of and take possession of his hand.
There are only 4 horcruxes left
By savingharry
The theory behind this is that Voldemort "used up" one horcrux when he died the first time (when he tried to kill harry). The piece of soul in one of the horcruxes was released and that is where "Vapormort," Voldemort without a body, came from. In this theory, horcruxes are "one-time-use only" items. Thus, with one horcrux used up and the ring and the diary destroyed, there are only 4 horcruxes left.
However, from what I can tell, a horcrux operates as a sort of anchor for the soul. By having a horcrux, not all of the soul can go on, as part of the soul remains hidden and safe. So the entire soul remains on earth, both the piece in the horcrux and the piece released when the person dies. Vapormort, in this case, is the piece of soul that was released from Voldemort’s body. The horcrux retains its soul piece, and is reusable for as long as the horcrux remains safe. Because of this, only one horcrux is needed to keep a person’s soul safe. Proof of this is the fact that no one, before Voldemort, thought of making more than one horcrux. Wouldn’t anyone willing to make one want more if they were used up on use like an extra life in a video game? Also, Riddle's diary seemed to be unaware of the path of its alter-ego after the time it was created. Remember that Ginny told it about Voldemort and Harry. Voldemort, however, knew about his own death and the events that preceded it. So that soul piece is the piece of soul that was in Voldemort before.
Why seven then? Voldemort split his soul into seven pieces (6 horcruxes and himself) because seven is the most magical number. Take the following:
----V----
-/-----\--
H------H-
|-------|-
H------H-
-\-----/--
---H-H---
Here, the peak is Voldemort himself, with the six other pieces as the six horcruxes. This makes a ring of seven that work together, magically strengthening the bonds between them, making the entire ring stronger. That is the purpose of Voldemort's seven horcruxes.
Horcrux hiding places relate to where they were taken from
By El Barto
Is there a significance between the Horcruxes and their finding places? Let's take, for example, the ones we currently know about (the locket, diary, Hufflepuff's goblet, and ring). The locket was found in a cave, hidden from the world, and shrouded in seemingly endless darkness and an unearthly green glow, with the dead likewise hidden from view the surface of the water which surround the small island on which it remained within a basin. Perhaps it shows that Voldemort, who fears love which a locket may represent, was placed in such a place which offers none. The diary wasn't necessarily found anywhere (wasn't in a hidden location), but what it represents is what may catch someone’s attention. People can write their secret thoughts and desires within the diary, relaying your emotions to what you think is an object that won’t judge you. Thus, it represents another place to shun love from the public, or if not love, then any emotion whatsoever. The goblet may be a bit tricky. A goblet can represent a fallacy, such as drinking, or something greater, like the Holy Grail. Perhaps the goblet is the ultimate representation of the hope of eternal life for Voldemort (and can potentially be found anywhere). The ring was found in the Gaunt House, and represents the strength of a family that has lasted several generation (or the crest, rather). Can the other Horcruxes be found in a similar fashion, symbolizing a long lost emotion or a strengthened, yet arrogant characteristic?
Each horcrux is taken from a different book
by kyp and savingharry
It seems that each of the horcruxes we know about has been revealed in one of the 6 books. Perhaps each horcrux will be revealed in each book:
Book 1 = Voldemort's Soul
Book 2 = Riddle's Diary (destroyed)
Book 3 = ?
Book 4 = Voldemort's snake, Nagini
Book 5 = The Locket in Sirius's house
Book 6 = Marvolo's ring (destroyed)
Book 7 = ?
This would mean that either one of the two remaining objects, either Hufflepuff's cup or the unknown object, or perhaps their location (such as the shrieking shack), would have been revealed in book 3.
The problem with this theory is that Hufflepuff's cup, if it is, indeed, a horcrux (and I see no reason to believe it is not), then it had to have been revealed in book 3, because you can’t say it was revealed in book 7, as we heard about it in book 6. I do not remember a cup casually mentioned in book 3 in the shrieking shack or anywhere else. So I do not see how this would work, in that sense. As well, the idea the shrieking shack as a horcrux location seems unlikely to me, because it would have needed to have been after Lupin left school, which was only 2 years before voldemort's downfall. I doubt Voldemort would have been able to have gotten so close to Hogwarts with Dumbledore as headmaster.
Remaining Horcrux Masterlist
This is the Masterlist of every suggestion I have found for horcruxes remaining to be identified.
What Dumbledore thinks is a horcrux:
Hufflepuff's Goblet
Nagini
Popular suggestions:
Harry Potter (or Harry's scar)
Gryffindor's Sword
Mrs. Weasley's tiara
Hogwarts
Mirror of Erised
Tom Riddle's Award for Special Service to the School
The Potters' graves in Godric’s Hollow
Godric Gryffindor's grave or a monument to him in Godric's Hollow
Something else from Godric's Hollow
Riddle's Wand
DADA office
Other suggestions:
Something in the Room of Requirement
Someone other than Harry (Quirrel, Ginny, Snape)
Tri-wizard cup
Cursed Necklace from Borgin and Burke's
Tom Riddle Sr.'s body/grave
Harry's Invisibility cloak
The Riddle house
One of the trophies taken while at the orphanage
Basilisk from the Chamber of Secrets
Harry's wand
Wormtail's silver hand
Sirius's motorcycle
One of the Two-way mirrors
Something Belonging to Moaning Myrtle
The Whomping Willow (though it was planted after Voldemort left school)
Tapestry containing Black Family History