Fiendfyre was used to assist in duels twice in the films. In the film adaptation of Harry Potter and the Order of the Phoenix, Lord Voldemort conjures a giant fire snake during his duel with Albus Dumbledore, who manages to hold off the flames.
The movie adaptations also display Lord Voldemort using Fiendfyre during his duel with Albus Dumbledore at the Ministry, though this never happens in the books.
Dumbledore fighting Voldemort in the Atrium of the Ministry of Magic. Voldemort attacks Dumbledore with a jet of green light (possibly Avada Kedavra), which Dumbledore -whilst non-verbally pushing Harry out of the way- counters with a cord of red light (possibly Expelliarmus or Stupefy).
Fiendfyre, or cursed fire, is an extremely dangerous and a dark piece of magic that wizards and witches like Hermione and Dumbledore would never have used. It also is pretty destructive: it set fire on the whole Room of Requirement and neither of them were unable to stop it.
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 screaming in pain before it was destroyed.
How Did Vincent Crabbe Cast Fiendfyre? - Harry Potter Theory
Why didn't Hermione use Fiendfyre?
Fiendfyre was one of the few substances known with the ability to destroy Horcruxes. 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.
When Crabbe cursed the Fiendfyre in the Requirement room (Goyle does it in the movie), he then dies by his own curse. If we believe the rules of the book, when the owner of a wand dies, the spells and curses casted before are broken as well.
It is also said to be a fairly complicated spell. It depends on a persons willingness to kill someone and also the power behind the wand. To quote Barty Crouch/Mad-Eye Moody in 'Harry Potter and the Goblet of Fire', “Avada Kadavra is a curse that needs a powerful bit of magic behind it.
Why doesn t Voldemort just use Avada Kedavra on Dumbledore?
In The Deathly Hallows Part 2, Voldemort meets Snape at the Boathouse and tells him the Elder Wand doesn't work properly for him. He concludes that, because Snape killed Dumbledore, the Elder Wand belongs to him. Voldemort knew he couldn't cast Avada Kedavra with the Elder Wand to kill its master.
Ultimately, it's a good thing that Dumbledore's plans didn't work out because if Draco hadn't disarmed the headmaster, Harry wouldn't have gained the Elder Wand's power when he later disarmed Draco. This was proof that, unlike Dumbledore, Harry was destined to reunite the Hallows and become Master of Death all along.
One of the three Unforgivable Curses outlawed under wizarding law, Avada Kedavra kills the victim on impact, instantaneously and painlessly, usually with a blinding flash of green light. There is no way to block the spell, and there is no counter-curse - it can only be dodged.
Snape questioned McGonagall as to why she was patrolling the corridors at that hour, and then discreetly used Legilimency on the Professor. As he asked McGonagall if she had seen Harry Potter, McGonagall quickly slashed her wand through the air, but Snape cast a Shield Charm that threw McGonagall off balance.
Harry used the Imperius Curse multiple times during the Gringotts heist to accomplish their mission. Harry never used the Killing Curse, Avada Kedavra, due to his moral stance against murder and his personal connection to its victims.
In the final movie, Harry Potter and the Deathly Hallows pt. 2, Molly first hits Bella with a spell that freezes her, most likely Petrificus Totalus. Then, she launches another one that shatters her entire body into pieces, which could be expelliarmus, bombarda, stupefy, or any spell that can produce such an effect.
Incredibly destructive, Fiendfyre often took the shape of enormous creatures such as serpents, dragons and birds of prey. The flames could not be extinguished by normal or enchanted water and once cast the curse was very difficult to control, multiplying and growing in power and strength.
Simple answer is, that's only in the movies. They often play fast and loose with things that might look or sound cool in the moment but make no sense in the overarching story. Ron never casts Avada Kedavra at Nagini in the books.
The intent of the caster is a vital component of the spell. While Snape was still able to produce Avada Kedavra without any intent to kill, being a great wizard, the mix of hesitation and necessity probably resulted in the spell being blue.
What happens if you use Avada Kedavra on a dementor?
From the Dementor's webpage: "They seemingly could not be destroyed, though their numbers could be limited if the conditions in which they multiply were prevented from forming, implying that they did die off eventually."
The author explicitly states that both women were fighting to kill, despite Molly refraining from casting the Unforgivable curse Avada Kedavra. The result of the duel actually comes about as a result of nothing more than spectacular accuracy on Weasley's part.
Why don't Death Eaters use Avada Kedavra more often?
Intense concentration is likely required to cast the Killing Curse, which is probably why Death Eaters don't use it as their primary offensive spell (although Thorfinn Rowle did launch the curse repeatedly all over the place during the Battle of the Astronomy Tower).
We all know Trelawney and Hagrid did the worst during Umbridge's inspection. (The Injustice of Hagrid's inspection still grinds my gears) And we know that Umbridge hates half breeds.
