Fiendfyre

"It was not normal fire; Crabbe had used a curse of which Harry had no knowledge: As they turned a corner the flames chased them as though they were alive, sentient, intent upon killing them. Now the fire was mutating, forming a gigantic pack of fiery beasts: Flaming serpents, chimaeras, and dragons rose and fell and rose again, and the detritus of centuries on which they were feeding was thrown up in the air into their fanged mouths, tossed high on clawed feet, before being consumed by the inferno."

- Description of Fiendfyre when Vincent Crabbe conjures it in the Room of Hidden Things during the Battle of Hogwarts

Fiendfyre is a type of fire made from Dark Magic. It is very powerful, as it cannot be put out by using the Aguamenti Charm (or any normal type of water), but is very difficult for the caster to control. The blaze is so potent that Fiendfyre is one of the very few substances that are capable of destroying a Horcrux.

Fiendfyre initially appears as a "roaring, billowing noise" and gives its victims only a moment's warning. Its flames are of abnormal size and take the shape of fire-breathing monsters and beasts, such as serpents, chimaeras, dragons, and raptors, continuously mutating. It appears to have enough consciousness to actually follow its intended victims. The fire will pursue nearby lifeforms as if it were alive itself, sentient, even the one who conjured it if they lack control. The fire will also pursue anything it can burn for more fuel and seems to incinerate anything through mere contact.

Inexperienced casters will be able to conjure it, but will have virtually no control over the flames once they are unleashed, which would make the flames a potential deadly backfire, though an experienced Dark Wizard (like Lord Voldemort) would be able to control it and manipulate it to great effect. When Fiendfyre (or any other magical fire) is allowed to burn unchecked for too long they can produce an Ashwinder.

Known uses
"Like it hot, scum?"

- Vincent Crabbe, as he conjures Fiendfyre in the Room of Requirement in an attempt to kill Harry Potter, but accidently kills himself at the Battle of Hogwarts

In 1998, Gregory Goyle learned how to cast Fiendfyre, presumably under the tutelage of Amycus Carrow, a Death Eater and professor of the Dark Arts at Hogwarts School of Witchcraft and Wizardry when it was under the control of Lord Voldemort. He was either not paying attention when Carrow discussed how to control the curse or lacked the skill to do so.

During the Battle of Hogwarts, Blaise Zabini, Gregory Goyle, and Draco Malfoy cornered Harry Potter, Ron Weasley, and Hermione Granger in the Room of Requirement during the latter group's search for Rowena Ravenclaw's Diadem. ,Gregory Goyle who at the time rebelled against Malfoy's leadership, conjured Fiendfyre in an attempt to kill Harry and his friends, but he was unable to control it. Since the fire was used in an enclosed area, Crabbe perished in the cursed blaze. Harry, Ron, and Hermione narrowly rescued Malfoy and Goyle before barely managing to escape.

When the diadem, having been exposed to the flames before its recovery, broke apart in Harry's hands upon their escape, Hermione realised it was Fiendfyre they had escaped from. She had known that it was capable of destroying Horcruxes, but claimed it was far too dangerous for her to attempt to use.

Etymology
The word "Fiendfyre" was probably derived from "fiend" (meaning an evil spirit or demon, The Devil) and fire.

Behind the scenes

 * Though the only confirmed use of this spell is by Crabbe/Goyle  in the Room of Requirement, there are numerous instances of living fire shown in the films that may also be Fiendfyre, or variants of it. These include a fire spell used by a Durmstrang student by blowing on the tip of his wand in Goblet of Fire, and a fiery snake utilized by Voldemort during his duel with Albus Dumbledore in the Ministry of Magic in Order of the Phoenix.
 * In Harry Potter and the Deathly Hallows: Part 2, Gregory Goyle uses Fiendfyre and dies instead of Crabbe. This is due to the fact that Crabbe has been cut from the film.
 * In the film Goyle dies by falling into the inferno after grabbing a loose chair rather than having the curse backfire as it did to Crabbe in the book.
 * In the film Fiendfyre is not used to completely destroy Rowena Ravenclaw's Diadem, instead a Basilisk fang is used to damage it, before the Fiendfyre spell engulfs it.

Appearances

 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows 
 * Harry Potter and the Deathly Hallows: Part 1 
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * LEGO Harry Potter: Years 5-7

Notes and references
Fiendfyre Dämonsfeuer Feudeymon Адский огонь Pirunpalo