In the film adaptation of
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.
Harry did not intentionally mean to hurt Malfoy to that point, when he acquired the book and saw the notes for the spell said it was for enemies. Thus he decided to try to use it on Malfoy. He was very guilt ridden and horrified by what he had done, and yes he definitely did get in trouble.
Harry cast Sectumsempra, true. In a duel that Malfoy instigated. In self-defence when Malfoy tried using the Cruciatus Curse. And Harry was severely punished.
Harry initially considered using it against Cormac McLaggen for annoying him, though it was not until he confronted Draco Malfoy in the sixth-floor boys' bathroom that he tested it, seriously wounding Malfoy.
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.
Sectumsempra is a curse invented by Professor Severus Snape, during his childhood, when he was known as "The Half-Blood Prince". He created it with the intention of using it against his enemies, and it soon became one of his specialties.
Voldemort killed Snape in Harry Potter And The Deathly Hallows Part 2 using his Horcrux snake Nagini instead of his favorite Unforgivable Curse, Avada Kedavra.
Snape used this spell on George Weasley in Harry Potter and the Deathly Hallows, cursing his ear off. Mrs. Weasley remarks that as his ear was severed by Dark magic, it will be impossible to grow it back. Mentioning that it was Snape who did it, Lupin remarks that "Sectumsempra was always a speciality of Snape's."
The Killing Curse ('Avada Kedavra')
Obviously there's no coming back from this spell, and according to the Ministry of Magic's classifications (and, again, fake-Moody) this is the worst Unforgivable Curse. If this countdown were about which spell is the most unforgivable we might agree – but weirdly enough it's not.
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.
Vulnera Sanentur was the song-like incantation of a healing spell which was used to heal deep wounds, which could also be used as the counter-curse for the curse Sectumsempra.
Blood spurts from Draco's face and chest, and Myrtle beings screaming. Snape bursts into the room and heals Draco's wounds almost immediately. Snape hurries Draco to the hospital wing, commanding Harry to wait behind.
History Of Avada Kedavra
The Killing Curse was created in the early Middle Ages by Dark witches and wizards. Primarily, the curse was used to quickly slay opponents during a duel.
It is possible that Snape used Legilimency to see through Harry's eyes, then quartered the Forest with his Patronus until Harry saw it.
I've always suspected Molly's lethal spell was a particularly well-placed and powerful stunner that stops Bellatrix's heart, which is consistent with the description of her death: Molly's curse soared beneath Bellatrix's outstretched arm and hit her squarely in the chest, directly over her heart.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
Harry Potter's use on Draco Malfoy in the Duelling Club. The Tickling Charm (Rictusempra), also known as the Rictusempra Charm, is a spell that causes a victim to buckle with laughter. It also has at least two other side effects on the victim. It is usually used as a diversionary tactic in duels.
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.
It is possible that this spell is Incendio Duo or Incendio Tria, given that both spells are used to create a large amount of intense flames, and that both are commonly used in Duels.
To keep his cover pretty much. Bellatrix never trusted him and this was her way of trying to prove she was right. There is also the deal with Dumbledore that he must be the one to kill him, so It was pretty much already in action to happen anyway.
Severus Snape: Doe
Image via Warner Bros. Similarly to Harry, Snape's patronus represents a person. In the memories he gives Harry before his death, Snape shows Dumbledore his patronus, using the form to prove his love of Lily Potter, Harry's mother.
In the final book, “Harry Potter and the Deathly Hallows,” Rowling reveals that Snape was acting on Dumbledore's orders and had protected Harry at school out of love for Harry's mother, Lily. “Snape died for Harry out of love for Lily,” Rowling said.