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.
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").
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.
Harry Potter World on Twitter: "6 May 1997: Harry used Sectumsempra on Draco Malfoy.
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.
For using Dark Magic, albeit unknowingly, to cause serious harm, Snape sentences Harry to detention every Saturday until school ends. Snape dismisses Harry's protests, as does Professor McGonagall, who later informs Harry that he was lucky not to be expelled.
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.
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.
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.
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.
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.
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.
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.
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.
And the third, the Killing Curse, is named by Hermione; Moody mentions that it can't be blocked, and only one person is known to have ever survived it: Harry Potter. Harry makes use of two of the Unforgivable Curses in the books.
McGonagall followed up Harry's action by using the Imperius Curse on Amycus before tying him up with a net, rendering him useless during the Battle of Hogwarts.
Sectumsempra | Harry Potter and the Half-Blood Prince - YouTube.
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.
The three Unforgivable Curses in Harry Potter are Crucio, Imperio, and Avada Kedavra, and all three are available to Hogwarts Legacy players. Crucio subjects the victim to extreme torture, while Imperio puts them under the control of the spellcaster.
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 Snape used Legilimency to see through Harry's eyes, then quartered the Forest with his Patronus until Harry saw it.
Harry's scar had hurt during Arrival Feast because of Quirrell's connection to Voldemort; Snape was then talking to Quirrell, allowing Voldemort, from the back of Quirrell's head, to sense Harry's presence.
She made sure HE was safe - putting THEIR LIVES at risk to save his - before they escaped. And yet while Bellatrix tortured Hermione, Draco did nothing.