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.
Sectumsempra is a very dark curse that causes a cut (Latin "sectum" = "cut") which will not heal ever (Latin "semper" = "always").
During Harry's hunt for a Horcrux, he accidentally attracted the attention of an army of Inferi, and in an attempt to stop them, he used many spells, Sectumsempra among them.
In LEGO Harry Potter: Years 5-7, Harry uses Sectumsempra on Malfoy only to find out the spell has, apparently, painlessly sliced him in half. In Harry Potter and the Deathly Hallows: Part 2, Voldemort may have used this spell to slit Snape's throat before having Nagini finish him off.
Snape knew that Harry had cast the spell on Malfoy because he had created the spell in the first place. He had probably used the spell many times and had even come up with a counter curse for it. It would be awfully sad if Snape couldn't tell when his own spell had been used on somebody.
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.
Vulnera Sanentur was the song-like incantation of a healing spell and counter-curse to the Sectumsempra Spell, which was invented by Severus Snape.
Blood spurts from Draco's face and chest, and Myrtle beings screaming. Snape bursts into the room and heals Draco's wounds almost immediately.
Therefore, figuring out a spell that is Draco's favorite is hard and, in the movies, there is no defining moment that cements a spell as his favorite. Based on Harry Potter lore, and an article by The Healthy Journal, Draco's signature spell is a N.E.W.T standard spell, The Protean Charm.
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.
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.
Snape's family background is mostly shown in flashbacks during the course of the last three novels. Snape was born to Eileen Prince, a witch, and Tobias Snape, a Muggle, making him a half-blood (hence the name, "Half-Blood Prince").
Snape's curse left a gash on James' face, but older James is never shown to have any scars, and wounds caused by dark magic aren't easily healed. As far as we are aware, Snape is the only one who knew the counter-curse to Sectumsempra, and he still said some scars might be left after he healed Malfoy.
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.
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.
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.
George Weasley and Remus Lupin arrived next, and Molly attempted to heal George's wound; he recovered, but his ear could not be healed because the wound was caused by Dark Magic.
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.
1 Avada Kedavra Is Just Ridiculous
Only love itself, the strongest magic of all, can block the Killing Curse. Lord Voldemort cast this spell constantly in his lifetime, and it's the spell that killed Harry's parents in particular.
Its incantation is Avada Kedavra. The only known counter-spell is sacrificial protection, which uses the magic of love.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
Voldemort told Snape he had been a faithful servant, but that he had to regrettably kill him, as he mistakenly believed he was the master of the Elder Wand, as Snape had killed Albus Dumbledore, its former owner the year prior, during the Battle of the Astronomy Tower.