Fidelius Charm

• Incantation: Unknown. The incantation for casting the Fidelius Charm is never stated in the novels.
• Pronunciation: Not applicable.
• Wand Movement: Unknown.
• Light: Unknown.
• Effect: Magically conceals a secret inside the soul of a single living person, who is designated the Secret Keeper. The information, often the location of a person or building, is rendered undiscoverable by any means—it becomes unplottable, invisible, intangible, and soundproof to anyone who has not been willingly told the secret by the Secret Keeper. The secret cannot be forced or tortured out of the Secret Keeper; it must be divulged voluntarily.
• Type: Charm (Extremely complex and powerful).

The Fidelius Charm is a powerful and ancient piece of magic, used in several critical instances throughout the First Wizarding War and Second Wizarding War.

• Potter Residence, Godric's Hollow: In 1981, James and Lily Potter went into hiding from Lord Voldemort under the protection of the Fidelius Charm. Albus Dumbledore advised them to use it and offered to be their Secret Keeper. However, they chose their friend, Sirius Black. At the last minute, Sirius persuaded the Potters to switch to Peter Pettigrew, believing Voldemort would never suspect someone as “weak and untalented” as Pettigrew. This was a fatal mistake, as Pettigrew was a spy for Voldemort and betrayed the Potters' location, leading directly to their deaths.
• Number Twelve, Grimmauld Place: The ancestral home of the Black family was used as the headquarters for the reconstituted Order of the Phoenix. Albus Dumbledore placed the building under a Fidelius Charm and made himself the Secret Keeper. To gain access, members of the Order were given a piece of parchment with the address handwritten by Dumbledore. Upon his death, a unique magical complication occurred: everyone Dumbledore had told the secret to became a Secret Keeper themselves, allowing them to pass the location on to others. This loophole ultimately compromised the headquarters when the Death Eater Yaxley caught hold of Hermione Granger as she Disapparated, allowing him to discover its location.
• The Burrow: Following the fall of the Ministry of Magic in 1997, the Weasley family home was put under a Fidelius Charm to protect it from Death Eaters. Arthur Weasley acted as the Secret Keeper.
• Shell Cottage: The seaside home of Bill Weasley and Fleur Delacour was also protected by a Fidelius Charm, with Bill serving as the Secret Keeper. He revealed the location to Harry Potter, Ron Weasley, and Hermione Granger so they could take refuge there after escaping from Malfoy Manor.

According to Professor Flitwick, the Fidelius Charm is “exceedingly difficult magic.” There is no known direct counter-spell; the charm's primary vulnerability is the human element—the trustworthiness of the chosen Secret Keeper.

• The Secret Keeper: The secret is entirely dependent on the Secret Keeper's loyalty. If the Secret Keeper voluntarily reveals the information, the charm is broken for the person who was told. Those who learn the secret are bound by the charm and cannot pass it on to others.
• Death of the Secret Keeper: If the Secret Keeper dies, as in the case of Albus Dumbledore, a unique magical state is created where every person who had been told the secret becomes a Secret Keeper. This significantly weakens the charm's integrity, as there are now multiple individuals who can divulge the secret.
• Forcible Entry: While a location under the Fidelius Charm cannot be found by outsiders, it is possible for an enemy to gain entry by accompanying someone who knows the secret during the moment of Apparition or Disapparition. This is how Yaxley was able to bypass the protections on Number Twelve, Grimmauld Place.

The name “Fidelius” is derived from the Latin word fidelis, meaning “faithful,” “loyal,” or “trustworthy.” This directly reflects the charm's fundamental reliance on the faithfulness and loyalty of the Secret Keeper.

• In the film adaptation of Harry Potter and the Order of the Phoenix, the effect of the Fidelius Charm on Number Twelve, Grimmauld Place is shown visually. The house magically pushes its way out from between numbers 11 and 13, becoming visible only after Harry has read the secret from the note provided by Dumbledore. (film)
• J.K. Rowling clarified on her website the magical principle that upon a Secret Keeper's death, everyone who knew the secret becomes a Secret Keeper. She explained this was a “convenient enchantment” that allowed more than a dozen people to know the secret of Grimmauld Place while Dumbledore was alive, and for them to pass it on after his death. (J.K. Rowling interview)