Hides a location
The Fidelius Charm is a complex and powerful Charm that takes a specific piece of information (a secret) and implants it into a being's soul. The one whose soul houses the secret is known as the Secret-Keeper.The purpose of this spell is to conceal a specific location, making it invisible, intangible, unplottable, and sound-proof.
Becoming Secret Keeper
When the charm is initially cast, one can choose more than one person to be Secret Keeper, though it is unknown if there is a limit as to how many people can carry the same secret in this way. Once the Secret Keeper dies, all the people who have learnt the secret from them will become the new Secret Keepers. If the Keeper has chosen not to reveal the secret to anyone (so that they are the only Keepers) then the secret rests with them forever.
Revealing the SecretOnce the secret has been implanted, the only method for another to learn it is for the primary Secret Keeper to divulge it to that person directly, it cannot be discovered any other way. The people who the Secret Keeper has already divulged the information to (secondary Keepers) would not be able to pass it along even if they wanted to and the information in question can not be learnt even by stumbling across it, even if one were staring the secret in the face. For example, if someone were to hide a location in this way, even if a person were to stumble into the specific area they would not be able to recognise it for what it was (i.e. as the area they were looking for) and they would not therefore learn the location accidentally.
Whilst one cannot force secondary Keepers to divulge the information they have learnt, it is unknown if coercive methods to have the primary Keeper reveal the location (such as Veritaserum or the Imperius Curse) would work.
The method for the primary Keeper to voluntarily tell another person the secret can be verbal or written. It is also possible that primary Keepers can reveal information to another by showing it to them. This theory is postulated because whilst 12 Grimmauld Place was under this charm and being used as a base of operations for Harry, Hermione and Ron only after Dumbledore's death, the Death Eaters evidently became aware of 12 Grimmauld Place's approximate location knowing that Harry Potter had inherited the property from Sirius Black. Despite 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. It is explained that because Hermione (along with everyone else who knew the house's location) had been rendered 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.
Once the secondary Keeper has received the information in either of these ways the information will still appear hidden to them until they start thinking about it, at which point the secret will reveal itself to them alone. For example, Alastor Moody gave Harry Potter a parchment with Albus Dumbledore instructing where the Order of the Phoenix's headquarters is. He then merely has to think about the said location whilst in the correct area and it would appear out of thin air, allowing him entry.
- 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; they 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.
- The Fidelius Charm was once more employed to cloak the base of the Order of the Phoenix at 12 Grimmauld Place. However, in 1997 when Albus Dumbledore (primary Keeper) was killed, the members of the Order of the Phoenix automatically became Secret-Keepers for the HQ. This included Severus Snape, whose allegiance was doubtful at the time. As such, the Order was forced to abandon their headquarters.
Behind the scenes
- Harry Potter and the Deathly Hallows contained a 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).
- 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 detectable, 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 a 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.
- 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 the location of Grimmauld Place. However, it was not until Hermione accidentally leading 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.
"Fidelis" is Latin, meaning trustworthy and faithful. "Fidelius" is the neuter comparitive form of this word, meaning more trustworthy and more faithful.