Fidelius Charm

The Fidelius Charm is a complex and powerful spell that allows someone or something to be hidden from the sight of anyone besides specific individuals for an indefinite period of time. Those specific people are the Secret-Keeper and the individuals to whom the Secret-Keeper reveals the information.

The secret being hidden is stored within the very soul of the Secret-Keeper. Only they have the power to reveal the secret; not even those who have been told the secret can reveal what they know. The information may be passed on through words or writing. If someone has not been given the information, they will be unable to locate the hidden secret, even if they are staring right at it. When the Secret-Keeper dies, everyone who has been told the secret becomes a Secret-Keeper. If the Secret-Keeper dies without having revealed the secret to anyone, then the secret dies with them, forever. The information will be hidden for all time (unless, of course, there were multiple secret-keepers before the death). It is unknown if coercive methods to have the Secret-Keeper reveal the location (such as Veritaserum or the Imperius Curse) would work.

The method of telling another person of the location by the Secret Keeper can be verbal or written, as Alastor Moody gave Harry Potter a parchment with Albus Dumbledore instructing where the Order of the Phoenix's headquarters is. Once the location is told, the recipient merely has to think about the said location, and it would appear out of thin air, allowing them entry. It is a protective spell.

Usage
The Fidelius Charm was used to protect James, Lily, and Harry Potter from Lord Voldemort at their home in Godric's Hollow. They designated their Secret-Keeper as Sirius Black, but then switched to Peter Pettigrew as they thought it would be less obvious; they had refused Albus Dumbledore's offer to be secret keeper due to them preferring their friends. Pettigrew betrayed them to Lord Voldemort, who found and murdered the Potters. Only Harry Potter survived the attack.

In 1997 when Albus Dumbledore was killed, the members of the Order of the Phoenix automatically became Secret-Keepers for their headquarters at 12 Grimmauld Place. This included Severus Snape, whose allegiance was doubtful at the time. As such, the Order was forced to abandon their headquarters.

Soon afterwards, the Death Eaters evidently became aware of 12 Grimmauld Place's approximate location knowing that Harry Potter had inherited the property from Sirius Black, as they took to watching the gap between numbers 11 and 13 when on the lookout for Harry. The house remained invisible to them until Yaxley grabbed onto Hermione Granger's arm while she, Harry, and Ron Weasley Disapparated away from the Ministry of Magic to 12 Grimmauld Place, where they had been hiding. Because Hermione had been rendered a Secret-Keeper by Dumbledore's death, she inadvertently took Yaxley past the protections of the Fidelius Charm. The trio subsequently abandoned the house as a hideout. Arthur and Bill Weasley used this charm on their households in 1997 when they went into hiding.

Behind the scenes
Harry Potter and the Deathly Hallows contained a few discrepancies regarding the Fidelius Charm. 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, 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. Although 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. This is logical since Bill and Arthur would not have known that they were in danger until after the first prisoners escaped from Malfoy Manor.

It is also conceivable that while anyone can know where people and places protected by a fidelius charm are located, once the charm is cast, only the secret keeper can reveal, as in, make visible, the location. This is backed up by the fact that Death Eaters knew that number 12 Grimmauld Place existed, and were stationed outside it, even though it was not visible to them. This alternate understanding of the word reveal makes it possible to understand number of these inconsistencies.

Also, the Fidelius Charm appears to be impervious to a Taboo Jinx, 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 without the Fidelius Charm being broken on 12 Grimmauld Place or even the Death Eaters appearing to be aware that the Taboo was crossed. 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. The Fidelius Charm seems, therefore, to not only render those residing in the charmed location as undetectable as the location itself, but does not transmit information which could counter the charm (such as speaking a Taboo name) outside its sphere of influence.

The working of everyone who has been told the secret becoming a Secret Keeper after the original one dies seems somewhat inconsistent, as when Dumbledore died, Snape should have become one, and told the Death Eaters the the location of Grimmauld Place. However, it was not until Hermione accidentally leading Yaxley to the place did the Death Eaters discover it. It remains possible however that due to Snape's true allegiance he simply chose not to reveal the location. Snape may also have been unable to reveal the location because of the Tongue-Tying Curse cast by Alastor Moody.

Etymology
"Fidelis" is Latin, meaning trustworthy and faithful.

Appearances

 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2

Sortilège de Fidelitas Uskollisuusloitsu