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

Fiendfyre was a bewitched flame of abnormal size and heat that could crumble fairly substantial objects to soot at a mere touch. The flames were infused with magic, capable of seeking out living targets despite being non-sentient. This curse is advanced dark magic, it is also very difficult to control the flames once they are unleashed.

Left burning long enough, the fire will take the shapes of gigantic fiery beasts (including serpents, chimaeras, dragons, and birds of prey).

Description and effects
"Fiendfyre - cursed fire — it’s one of the substances that destroy Horcruxes, but I would never, ever have dared use it, it’s too dangerous."

- The dangers of the spell

Fiendfyre is an immensely powerful fire that cannot be extinguished by normal or enchanted water, It is also very difficult for the caster to control, flowing from their wand in a continuous stream of flame. If the caster flicks their wand when the stream of flame is still running from it, a jet of fire will shoot off and become a flaming animal.

When cast, the fire appears with a roaring, billowing noise and gives its victims only a split-second's warning to escape, quickly consuming anything in the vicinity of its caster. The flames are of an abnormally large size and take the shape of fiery monsters and beasts such as snakes, dragons, eagles, and chimeras, constantly mutating into other beasts as well as powerful, formless flames that destroy all things flammable around itself. The fire even possesses a sentience of its own, a continuous (though far from latent) desire to burn anything it can. It will pursue any nearby lifeforms and anything that it can destroy, and is capable of incinerating anything through mere contact.

Inexperienced casters will be able to conjure the flames but will have virtually no control over the curse once it has been unleashed, thereby making the fire a deadly backfire. There is also a charm to cease the flames, but unfortunately Vincent Crabbe never paid attention in class long enough to learn them, therefore costing him his life. It is also possible that the fire may eventually burn off on its own, if it is unable to consume enough material to sustain it, and if so, the ash left behind may become Ashwinders.

Fiendfyre was one of the few substances known to be able to destroy a Horcrux. Hermione Granger was aware of this, but never considered the use of it against Voldemort's Horcruxes due to the inherently dangerous, uncontrollable nature of the spell.

Fiendfyre can be cast without using a wand. a common example is Lord Voldermort summoning a Fiendfyre in a form of a snake to attack Albus Dumbledore during the Battle at the Department of Mysteries.

Known practitioners
"Bloody hell! Goyle set the whole bloody room on fire!"

- Ronald Weasley

Behind the scenes

 * In  Gregory Goyle uses Fiendfyre and dies instead of Crabbe, due to the fact that Crabbe had been cut from the film. Also, whereas Crabbe died because he could not run fast enough from the fire, Goyle is depicted as having been unable to stop the flame from pouring out of his wand (who then ends up throwing it away into the flames), and falling into the flames from grabbing a loose chair while climbing up a mountain of various objects to safety.
 * In the film adaptation, Fiendfyre was not used to completely destroy Rowena Ravenclaw's Diadem. Instead, Harry stabbed it with the Serpent of Slytherin's fang to damage it, and then Ron kicked it into the Fiendfyre to finish the job. This led Voldemort's mangled soul to possess the flames briefly and scream in pain before it was destroyed.
 * Fiendfyre is used by a Durmstrang student at the beginning of the Triwizard Tournament, mostly for show. Fiendfyre is also used to assist in duels twice in the films. Lord Voldemort conjures a giant fire snake during his duel with Albus Dumbledore, whose own spellwork counters the attempt on his life. Likewise, Bellatrix Lestrange uses Fiendfyre to destroy the Burrow when the Death Eaters attempted to capture Harry Potter on Christmas in 1996. However, both events contradict the events of the books. In the books, Dumbledore forces Voldemort to conjure a shield, and Rufus Scrimgeour visits the Burrow instead of Bellatrix and Fenrir Greyback. Therefore, of these three acts of Dark magic, none can be considered canon yet within the continuity of the books.
 * The curse resembles a magical fuel-air explosive or thermobaric bomb.
 * The protective dark charm Protego Diabolica is remarkably similar in its affects to Fiendfyre, with the notable exception of the former being harmless on those allied with the castor.

Notes and references
Fiendfyre Dämonsfeuer Feudeymon Адское пламя Pirunpalo Szatańska Pożoga pt-br:Fogomaldito