Fiendfyre

"Like it hot, scum?"

- Vincent Crabbe, as he conjures Fiendfyre in the Room of Requirement in an attempt to kill Harry Potter

Fiendfyre (also known as Cursed Fire) is a type of fire made from Dark Magic. It is very powerful, as it is capable of destroying Horcruxes and cannot be put out by Aguamenti, but is very difficult for the caster to control.

Fiendfyre initially appears a "roaring, billowing noise" and gives its victims only a moments warning. Its flames are of abnormal size and take the shape of monsters and beasts, such as serpents, Chimaeras, and dragons, continuously mutating. The fire will pursue nearby lifeforms as if it were alive itself, sentient and intent upon killing. The fire will also pursue anything it can burn for more fuel.

It is unknown if there is a counter-curse for Fiendfyre; Aguamenti merely evaporates before touching it. Inexperienced casters will be able to conjure it, but will have virtually no control over the flames once they are unleashed, though an experieced Dark Wizard would be able to manipulate it to great effect.

Known Uses
In 1998, Vincent Crabbe 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, Crabbe, 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. Crabbe, 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. Because 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.

Behind the scenes

 * In Harry Potter and the Goblet of Fire (film), A student from Durmstrang casts a fiery creature upon entering the Great Hall. However, he casts the spell by blowing upon his wand which is not explicated in the books. Thus, it is unlikely that the spell was Fiendfyre.
 * In Harry Potter and the Order of the Phoenix (film), Lord Voldemort, while in a duel with Albus Dumbledore, summons what appears to be Fiendfyre from the known description. However, it is currently unknown if this spell is meant to be a representation of the spell, or of a different spell entirely.