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.
Sectumsempra is a very dark curse that causes a cut (Latin "sectum" = "cut") which will not heal ever (Latin "semper" = "always").
Harry Potter World on Twitter: "6 May 1997: Harry used Sectumsempra on Draco Malfoy.
Harry rushes to the bathroom where Draco is crying at the sink, sobbing that unless he does something soon, he will be killed. Draco spots Harry and pulls out his wand. Harry yells “Sectumsempra,” the hex scribbled in the margins of the Half-Blood Prince's book.
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…'
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.
Sectumsempra | Harry Potter and the Half-Blood Prince - YouTube.
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.
Although Harry was justifiably defending himself against Malfoy, who was about to cast the Cruciatus Curse, using an unknown spell was dangerous and reckless. However, despite this grievous action that could have permanently expelled him from Hogwarts, Snape only assigns Harry detention.
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.
Rictusempra, the Perpetual Tickling charm, which apparently can be stopped by the Finite Incantatem charm, was used by Harry against Malfoy in the Duelling Club. The intent was, of course, to tickle Malfoy to the point that he would be unable to jinx Harry in return.
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.
It is possible that Snape used Legilimency to see through Harry's eyes, then quartered the Forest with his Patronus until Harry saw it.
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.
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.
Quirrell didn't use the Killing Curse on Harry, because Voldemort said “use the boy” to get the Sorcerer's stone from the Mirror of Erised. If Quirrell had killed Harry right away, he would have lost the chance to get the stone.
Hermione's bluebell flames
Hermione first uses it against Snape when she and Ron think he is cursing Harry's broom in his first Quidditch match. Her bright blue flames set Snape's robes alight, before Hermione scoops them up and puts them into a jar.
It's unknown quite how they managed this, although we do know that certain spells do levitate things, such as the Levitation Charm, or Snape's self-penned spell, Levicorpus.
He finds out for sure immediately after Nagini's attack on Arthur Weasley. This night is significant in many ways. While Harry sleeps, he enters Nagini's mind.
Used it to sever the hand of a fellow Death Eater, who had been aiming at Remus Lupin's back during the Battle of the Seven Potters, but due to a slip, he accidentally cut off George's left ear instead, leaving him guilt-ridden.
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.