Entirely Fidelius Charm

The Fidelius Charm was a very powerful spell used to hide things or places, or to conceal secrets. It was an extremely difficult, multifaceted, and potent charm that could be used to conceal a secret inside an individual's soul. The witch or wizard who housed the secret was known as the Secret Keeper. A dwelling whose location had been protected by this spell was then invisible, intangible, unplottable, and soundproof. This was an extremely old spell, one of the most ancient of all.
==History==
===First Wizarding War===
The Fidelius Charm was used to protect James, Lily, and Harry Potter from Lord Voldemort at their home in Godric's Hollow, the location of which was the secret entrusted. They designated their Secret-Keeper as Sirius Black, but then switched to Peter Pettigrew at the last minute before the spell was cast as they thought it would be less obvious.
Lily and James had refused Albus Dumbledore's offer to be secret keeper due to them preferring their friends, which proved a deadly mistake — Pettigrew betrayed them to Lord Voldemort, who found and murdered the Potters, with only Harry Potter surviving the attack. Harry's survival would henceforth dub him the "Boy Who Lived".
==Nature==
===Becoming Secret Keeper===
When the charm was initially cast, one could choose more than one person to be Secret Keeper, though it is unknown if there was a limit as to how many people could carry the same secret in this way. Once the Secret Keeper died, all the people who had learnt the secret from them would become the new Secret Keepers. If the Keeper had chosen not to reveal the secret to anyone (so that they are the only Keepers), then the secret rested with them forever.
It is unknown what criteria had to be met before someone could qualify as a Secret Keeper. However, it appeared that, to be eligible, one had to be capable of divulging the secret if they wished to, as this would explain why wizards did not simply bestow the secret onto creatures who could not speak a human language, or creatures such as house-elves, who could simply be ordered by their masters to not divulge the secret (as such an order would override their free will). It was also possible that the spell had to be cast by the Secret Keeper him/herself, so only a wizard can be it.
===Revealing the secret===
==Known uses==
Caster(s)	Date 	Secret concealed	Secret Keeper(s)	Notes
Unknown 	Late October 1981	Potter cottage 	Peter Pettigrew 	Used to protect James, Lily, and Harry Potter from Lord Voldemort.
==Etymology==
"Fidelis" is Latin, meaning trustworthy and faithful. "Fidelius" is the neuter comparative form of this word, meaning more trustworthy and more faithful. "Fidelius" can also translate to authors.
==Behind the scenes==
*It's very likely that all people who already knew the information before the spell is cast automatically become secondary keepers upon casting, even if they are unaware of the fact. That renders them incapable of sharing the information, but they still remember it themselves. Otherwise, the Fidelius Charm could be used as a powerful brainwashing spell that affects all living humans, up to making well-known facts forgotten and impossible to rediscover. This explains why Dumbledore immediately knew where Hagrid should look for Harry after his parents' deaths, even though Dumbledore had not been told the location of the Potters' house by its Secret Keeper.
*Few notable discrepancies regarding the Fidelius Charm. For example, it is not known whether or not Yaxley had learned the location of 12 Grimmauld Place by "piggybacking" on Hermione, or even if he had comprehended where he had been taken by Hermione; he may simply have stumbled out of the hideout unaware of his true location between exiting and entering it.
*When the Weasley family was forced into hiding, Bill told them that Arthur was Secret-Keeper and also told them that they were staying at Aunt Muriel's. Since Arthur was Secret-Keeper, it would logically follow that Bill should not have been able to tell the trio about the Weasleys' whereabouts. Also, when Harry, Ron, and Hermione were captured and taken to Malfoy Manor, Ron told Dobby where to take the hostages when the house-elf came to rescue them. There are several theories to explain these incidences: whilst it is possible that Bill and Arthur were both Secret-Keepers for Muriel's, and that Ron had been made into an additional Secret-Keeper for Shell Cottage, this was never specified. It is more likely that the Fidelius Charms were cast after the trio arrived at the cottage, since Bill and Arthur would not have known that they were in immediate danger until after the first group of escaped prisoners arrived at Shell Cottage. Or perhaps it is due to the fact that elf magic is beyond that of wizard magic (i.e. being able to apparate in/out of Hogwarts, being able to apparate in/out of Malfoy Manor). However, if house elves can bring people into protected places without permission, anyone can ask a house-elf to break into any places if they own that house-elf.
*Also, the Fidelius Charm appears to keep the secret location hidden even upon a Taboo Jinx being triggered within it, such as the one placed on Lord Voldemort's name after the Fall of the Ministry of Magic. Several times, unaware of the Taboo, Harry and Hermione say Voldemort's name on 12 Grimmauld Place, yet when the Death Eaters appear in the square outside, they are still not able to see the house. This directly contrasts the result of Harry saying the name while in the tent, which was followed immediately by the enchantments and charms placed there disappearing and a team of Snatchers Apparating outside.
*It is also worth noting that, when Dumbledore died, Snape would have become a primary Secret-Keeper. Therefore, he should have theoretically been able to tell the Death Eaters the location of Grimmauld Place. However, it was not until Hermione accidentally leading Corban Yaxley to the place did the Death Eaters (possibly) discover it. Snape either must've chosen not to reveal the location because of his secret loyalty to the Order, or may have been unable to reveal the location because of the Tongue-Tying Curse cast by Alastor Moody. Neither of these theories are ideal, though, since Voldemort would likely have tortured, or even killed, Snape had he declined to reveal the location (however, this wouldn't have worked, as the secret must be revealed willingly, as mentioned below), and if a tongue-tying curse was used to keep Snape from talking, that would kill the theory mentioned above that, to be secret-keeper, one must be capable of revealing the secret if they choose to (which, in turn, would beg the question as to why only humans, and not house elves or common animals, are ever chosen as secret-keepers).
**Another theory is that Voldemort never knew that Albus Dumbledore was the Secret Keeper, and thus did not know Snape became one of the Secret Keepers after Dumbledore's death and hence not suspect Snape. After all, the only time Snape is ever shown discussing the Fidelius Charm with other Death Eaters is in Book 6, Chapter 2 ("Spinner's End"). In that conversation, Snape never says that Dumbledore is the Secret-Keeper, only that he himself is not. If probed for more details, perhaps Snape could have lied and simply said he only received an anonymous letter, whose handwriting he did not recognize, telling him the location before having the letter immediately be destroyed, much like how Harry receives the information during Order of the Phoenix.
**Another possibility is that Voldemort had decided it would be worthless to enter Grimmauld Place since the Order would have changed headquarters. However, this is unlikely, as it was still a place with strong ties to the Order, and it was owned by Harry Potter.
*Rowling once stated that, should the Secret Keeper die, anyone who knew the secret would be permanently unable to pass it on.FAQ on Rowling's official site She later revised this after Order of the Phoenix was published, and in Deathly Hallows, it was made explicit that when a Secret Keeper dies, everyone who knew the secret becomes a Secret Keeper.
*It should be noted that even the inhabitants of the secret location can become a Secret Keeper, so one must wonder why James or Lily didn't become their own Secret Keeper, instead of having their friends become one which would have effectively made them a prime target for Voldemort. But this can be explained by what Flitwick says in the third book: "not even if he had his nose pressed against their sitting room window!". Suggesting that in the Potters' case not the location but James and Lily themselves are the secret. Meaning that when one of them was the secret keeper nobody would be able to see or hear them and they wouldn't be able to disclose their secret to anyone else.
*The Potters' house was the secret. Lily mentioned in her letter that James had grown tired of being cooped up in the house. They needn't have stayed in the house if they themselves had been the secret. It's also said the charm was cast on their house.
*It's unclear why Hermione didn't use the Fidelius Charm to prevent Professor Umbridge from discovering Dumbledore's Army. While the spell is mentioned to be 'immensely complex' and so it is possible that it may have been beyond her abilities, she was able to cast a Protean Charm on the coins that she issued to the group despite that spell being NEWT standard, so it's not implausible that she could've been able to do a Fidelius Charm.
*Sirius persuaded Lily and James to use Peter Pettigrew as Secret Keeper as a decoy, believing that he himself would be too obvious a choice and thus a prime target for Voldemort. However, as a Secret Keeper can only reveal the information willingly and Sirius was so adamant that he would never have betrayed the Potters, this move appears somewhat unnecessary. If Sirius was Secret-Keeper, targeted, and killed, each of the people to whom the location had been confided would become Secret-Keepers in turn, which could increase the number of people who could reveal the location to Voldemort.
*It is unknown whether someone who has split his/her soul via a Horcrux can become a Secret Keeper, or how the secret-keeping would be affected if they can. It is also unknown what would happen if a Secret Keeper were to receive the Dementor's Kiss.
===Mentions in the book===
*"an immensely complex spell ... involving the magical concealment of a secret inside a living soul. The information is hidden inside the chosen person, or Secret-Keeper, and is henceforth impossible to find -- unless, of course, the Secret-Keeper chooses to divulge it." Harry Potter and the Prisoner of Azkaban
*"Harry --" "Look... Look at it, Hermione ..." "I don't ... oh!" He could see it; the Fidelius Charm must have died with James and Lily. Harry Potter and the Deathly Hallows
*"Dumbledore's Secret-Keeper for the Order, you know -- nobody can find Headquarters unless he tells them personally where it is" Harry Potter and the Order of the Phoenix
*Mr Weasley had explained that after the death of Dumbledore, their Secret-Keeper, each of the people to whom Dumbledore had confided Grimmauld Place's location had become a Secret-Keeper in turn. "And as there are around 20 of us, that greatly dilutes the power of the Fidelius Charm." Harry Potter and the Deathly Hallows
*"As we Disapparated, Yaxley caught hold of me ... we arrived at Grimmauld Place ... I've already taken him inside the Fidelius Charm's protection. Since Dumbledore died, we're Secret-Keepers, so I've given him the secret, haven't I?" There was no pretending; Harry was sure she was right. It was a serious blow. If Yaxley could now get inside the house, there was no way that they could return. Even now he could be bringing other Death Eaters in there by Apparition Harry Potter and the Deathly Hallows
*"...The Death Eater's know Ron's with you now, they're bound to target the family" ... "How are they protected?" asked Harry. "Fidelius Charm. Dad's Secret-Keeper. And we've done it on this cottage too; I'm Secret-Keeper here." Harry Potter and the Deathly Hallows
==Appearances==
==Notes and references==
Category:Concealment
Category:Known Protective Spells
Category:Entire Soul-Related Magic
Category:Spells Of-Unknown Incantation