Cruciatus Curse ('Crucio') Harry tried to use this spell himself. The first time was against Bellatrix after she had killed Sirius. His attempt was unsuccessful because, as Bellatrix mockingly explained: 'You need to really want to cause pain – to enjoy it – righteous anger won't hurt me for long…'
For example, despite being furious with Bellatrix Lestrange for her murder of his godfather in 1996, Harry Potter was only able to cause her a brief moment of pain with the Cruciatus Curse because he used it out of "righteous anger", though he was still able to knock her off of her feet with it.
Harry makes use of two of the Unforgivable Curses in the books. Dueling with Bellatrix Lestrange, he attempts the Cruciatus curse, with limited results; Bellatrix says that he has to really hate someone to make the Unforgivable Curses work properly, righteous indignation isn't enough.
The darkest of the Dark Arts, Harry Potter unforgivable curses are the Cruciatus Curse, the Imperius Curse, and the Killing Curse. Although the curses are highly illegal, Harry using Unforgivable Curses happened, but he uses them once he was thrust into the Second Wizarding War.
Harry wasn't arrested for the simple reason that the people he used it on, Bellatrix Lestrange, Severus Snape and Amycus Carrow were in no position to make a complaint. Bellatrix was an escapee from Azkaban Prison who had just committed a murder in the Ministry of Magic.
Voldemort would dodge it. That's the simple answer. There's no way they could hit him with it. Harry did try to use the killing curse in movie 5 against Bellatrix who stated something to the effect he couldn't do it because he really had to mean it, implying it had to be in his soul to want to kill people.
There is likely a good reason that the Sectumsempra is not an unforgivable curse. It's mostly because Sectumsempra is Snape's personal curse that he created. It is not a widely known or used spell, and it is one that he kept secret — which means it can't be added to the list since no one really knows about it.
They were first classified as "Unforgivable" in 1717. They are the Killing Curse, Avada Kedavra, the Cruciatus Curse, Crucio, and the Imperius Curse, Imperio.
Harry became so enamoured with the spell that he eventually used it to finish off Lord Voldemort. Here's how Harry's relationship with Expelliarmus became his signature – and why that's a good and bad thing.
Draco Malfoy On Harry Potter (Cruciatus Curse)
During this fight, Draco attempts to use the Cruciatus Curse - only for it to miss its mark. Harry actually comes off looking like the bad guy after using Sectumsempra, a spell he'd read about in the Half-Blood Prince's own potion book.
Ron never cast any kind of curse on Nagini, not in the book anyways. And there's no confirmation that the spell he used on her in the movies was the Killing Curse. Avada Kedavra isn't the only spell that creates green light, after all.
Malfoy spots Harry and casts a Cruciatus curse; Harry, defending himself, using the Half-Blood Prince's Sectumsempra spell without knowing its effects. To his horror, it gashes Draco's face and chest, spilling his blood everywhere. Moaning Myrtle flies off, screaming.
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.
In Deathly Hallows, Hermione faced a real test of her physical and psychological stamina. After being on the run from a bunch of Snatchers, the gang were taken to Malfoy Manor, where Bellatrix proceeded to torture Hermione using the Cruciatus Curse.
So, Voldemort had to kill Snape for the wand's allegiance to switch. Voldemort knew he couldn't cast Avada Kedavra with the Elder Wand to kill its master, so he ordered Nagini to be the one who killed Snape. In reality, the Elder Wand belonged to Draco, as he was the one to disarm Dumbledore in the Astronomy Tower.
Its incantation is Avada Kedavra. The only known counter-spell is sacrificial protection, which uses the magic of love. However, one may dodge the green bolt or block it with a physical barrier.
Avada Kedavra
Topping the list for the most powerful spells in the Harry Potter universe is no other than Avada Kedavra, Voldemort's favorite to instill his will and fear onto others. The spell itself is a play on Abra Kadabra and is easy to cast by just about anyone in the Wizarding World.
The Killing Curse, is straightforward in that it instantly inflicts painless death on the victim. Harry used two unforgivable curses, and each occasion weighed heavily on him.
The Patronus Charm is perhaps the most notorious difficult-to-cast spell in the wizarding world, and it's certainly the first one to come to mind for most fans. The Patronus Charm requires the caster to hold on to a moment of pure happiness—as in, they must think of their happiest memory.
He suffers an injury from a Sectumsempra curse from Severus Snape (we find out too late that Snape didn't actually mean to injure George, and was instead trying to discreetly hit another Death Eater).
He is also revealed to be a double agent for Albus Dumbledore and the Order of the Phoenix. But many people forget that he also invented a number of spells. In the Harry Potter movies, Snape came up with the Sectumsempra curse when he was a student at Hogwarts, and it's known to cause painful lacerations on the target.
As vile as this fight was, the question remains whether Snape gashed James' face using Sectumsempra. The short answer is no. While yes Snape did gash the side of James' face, Sectumsempra is a far deadlier curse.