During Harry's Occlumency lessons with Snape in
The Legilimency Spell (Legilimens) allows the caster to perform Legilimency; in other words, to delve into the mind of the victim, permitting the caster to see memories, emotions and thoughts.
Snape used the Legilimens (mind-reading) spell on Harry, and Harry used the Protego Charm, sending it rebounding on Snape. Basically, Harry caused Snape's mind-reading spell to boomerang onto him so harry was able to use the spell and not Snape.
The curse was invented by Severus Snape and recorded during his time as a student at Hogwarts, when he was known as "The Half-Blood Prince". He created it to retaliate against his enemies such as the Marauders, and he recorded it in his N.E.W.T. -level Potions textbook.
In Deathly Hallows, Harry finally masters occlumency – shutting his mind to Voldemort – when Dobby dies. He realises that his grief – or as Dumbledore calls it, love – can block out the Dark Lord.
Draco the Occlumens
There was, however, one Hogwarts student who mastered Occlumency: Draco Malfoy. Occlumency allowed Draco to pursue his secret mission throughout his sixth year, apparently undetected. Draco mastered Occlumency with help from his aunt Bellatrix.
Albus Dumbledore was also an exceptional Occlumens, having been one of the few to be capable of repelling Voldemort's Legilimency and he also expressed confidence in his ability to teach Harry the art but chose not to due to fearing Voldemort's reaction to how close they were.
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.
The short answer is no. While yes Snape did gash the side of James' face, Sectumsempra is a far deadlier curse. When Harry used the hex in The Half-Blood Prince, Draco collapsed onto the floor, and severely bleed through his face and chest.
For using Dark Magic, albeit unknowingly, to cause serious harm, Snape sentences Harry to detention every Saturday until school ends.
It's a word that gained so much more clarity and tragedy after Harry Potter and the Deathly Hallows revealed that the nefarious Professor Severus Snape was actually serving a nobler cause: protecting Harry from Voldemort out of love for Snape's childhood friend Lily Evans.
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.
He said, 'According to Victorian flower language, asphodel is a type of lily meaning “my regrets follow you to the grave” and wormwood means “absence” and also typically symbolises bitter sorrow. If you combined that, Snape's words mean “I bitterly regret Lily's death”.
Exploring Harry Potter's signature spell. The spell became Harry's trademark.
Despite all the access Snape has to Harry's mind, their mutual hatred colors the information Snape receives and makes him resistant to facts that don't confirm his previous beliefs.
Because he knew how to compartmentalise
Occlumency is about partitioning up and blocking off parts of your brain, something Snape knew all about. Even before he started working against Voldemort, he'd spent years putting his emotions in boxes.
James isn't motivated by anything much in attacking Snape in this way; he does it because he's bored, because he can, and because he wants to show off to some admiring girls, including Harry's mother, Lily. Harry is deeply disturbed by witnessing Snape's worst memory.
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.
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.
How Expelliarmus Beat Voldemort's Avada Kedavra - Harry Potter Explained - YouTube.
When the time comes, and Snape is facing Dumbledore with his wand out at the top of the Astronomy tower- he finally releases the spell that ends Dumbledore. We can very clearly see that Snape used the most effective method of killing known to the Harry Potter story- avada kedavra, the killing curse.
Harry yells “Sectumsempra,” the hex scribbled in the margins of the Half-Blood Prince's book. 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.
Lord Voldemort (Ralph Fiennes), aka Tom Riddle, was described by Dumbledore as one of the best students ever to graduate Hogwarts as well as "the most dangerous Dark wizard of all time." Before he had even learned about the existence of the Wizarding world, he was terrorizing his fellow orphans with impressive but ...