Fiendfyre

• Incantation: Unknown. Vincent Crabbe cast the spell by yelling a phrase that Harry Potter did not understand.
• Pronunciation: N/A
• Wand Movement: Unknown
• Light: N/A (Manifests as vast, physical flames)
• Effect: Conjures a torrent of semi-sentient, bewitched fire that takes the form of monstrous creatures such as giant serpents, chimaeras, and dragons. The flames actively and aggressively seek out living beings and will incinerate nearly any object in their path. It is one of the few known substances capable of destroying a Horcrux.
• Type: Dark Magic (Curse)

Fiendfyre is an extremely dangerous and powerful form of Dark Magic. Its destructive capabilities were well-known, as Hermione Granger identified it as one of the potential methods for destroying Lord Voldemort's Horcruxes, though she was terrified of the prospect due to the curse's uncontrollable nature. The most significant use of Fiendfyre occurred during the Battle of Hogwarts. Inside the Room of Requirement, Vincent Crabbe, in an attempt to kill Harry Potter, Ron Weasley, and Hermione Granger, unleashed the curse. Crabbe, despite being taught how to cast the spell by the Carrow siblings, lacked the magical skill to control it. The fire quickly spiraled out of his command, consuming the entire room in a roaring inferno of fiery beasts. The consequences of this act were severe:

• The cursed flames destroyed the hidden form of the Room of Requirement, incinerating centuries of lost items stored within.
• It unintentionally destroyed one of Lord Voldemort's Horcruxes, the Ravenclaw's Diadem, which Harry had just located.
• Vincent Crabbe was killed by his own spell, unable to escape the conflagration he had created.
• Harry Potter managed to rescue Draco Malfoy and Gregory Goyle from the fire before escaping with Ron and Hermione on two old broomsticks.

Fiendfyre is considered exceptionally difficult and advanced Dark Magic. As demonstrated by Vincent Crabbe, simply knowing how to cast the spell is not enough; controlling it requires immense magical power and discipline. A caster who loses control of the fire is very likely to become its victim. There is no known counter-spell or direct method to extinguish Fiendfyre once it has been unleashed. Its immense power is such that it can overcome most conventional magical protections. The fire in the Room of Requirement was only contained when the enchanted door to the room vanished, trapping the inferno inside until it presumably burned itself out.

The name Fiendfyre is a compound of two words:

• Fiend: An English word for a demon, devil, or an evil and cruel person. This refers to the malevolent, sentient nature of the flames.
• Fyre: An archaic, Middle English spelling of “fire.”

Together, the name literally means “Demon Fire,” an apt description for a curse that manifests as a living, malicious inferno.

• In the film adaptation Harry Potter and the Deathly Hallows – Part 2, the roles are reversed. Gregory Goyle is the one who casts Fiendfyre and is subsequently killed by it, while Vincent Crabbe's role is reduced. Blaise Zabini is present instead of Crabbe. (film)
• Fiendfyre is a usable spell in the LEGO Harry Potter: Years 5-7 video game, where it is used to destroy specific silver objects. (video game)