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