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.
Vulnera Sanentur was the song-like incantation of a healing spell and counter-curse to the Sectumsempra Spell, which was invented by Severus Snape.
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.
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.
It is believed that perhaps it was Snape who informed him about the plan, seeing as Snape's true loyalty lied with Dumbledore. Preferring not to die at Draco's hands and knowing Draco's life was at stake should he fail, Dumbledore enlisted Snape's help to do the deed instead.
In short, Snape favors Draco because Lucius was his old friend and Snape has a rather difficult time separating children from their parents.
Snape even said that he didn't want anyone to find out that he was protecting Lily's child, “especially Potter's son” (pg. 679 of Harry Potter and the Deathly Hallows) because of his hatred for James. Instead of providing Harry a safe haven from his abusive home with the Dursleys, he tormented Harry over an old grudge.
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.
For using Dark Magic, albeit unknowingly, to cause serious harm, Snape sentences Harry to detention every Saturday until school ends.
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.
While Snape was engaged in combat, his goal was not to harm George; instead, he was trying to save Remus Lupin from a fellow Death Eater's Avada Kedavra curse. This misdirection of Sectumsempra speaks to a lack of complete control in a battlefield scenario.
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. However, this merely slashed their unfeeling flesh, causing no impediment at the very least.
Most likely it was because he was the one who created the curse to begin with. If he hadn't wrote that spell into the book Harry wouldn't have used it on Malfoy to begin with.
Snape apparently told Draco to conjure the snake which I find rather odd. This is not exactly a good dueling spell.
The dueling club meeting serves as a catalyst for the school to suspect Harry, and it is debatable whether Snape instructed Malfoy to conjure a snake simply because it is the Slytherin mascot, or because he wanted to test whether Harry could be the heir by seeing whether he could communicate with snakes.
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.
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.
Cruciatus Curse ('Crucio')
Repeated use of the Cruciatus Curse is torture and Bellatrix Lestrange subjected the Longbottoms to such extremes of pain that it sent them insane. 'Better dead than what happened to them,' was what Moody said of the Longbottoms' fate, and there's a sad ring of truth to that statement.
For example, despite being furious with Bellatrix Lestrange for her murder of his godfather in 1996, Harry Potter was only able to cause her a brief moment of pain with the Cruciatus Curse because he used it out of "righteous anger", though he was still able to knock her off of her feet with it.
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.
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.
Now, as firmly established, Snape was not the greatest fan of Harry, but that didn't mean that he ever stopped loving Lily. Dumbledore was surprised that Snape seemed to care for the boy. With a swish of his wand, Snape conjured up a Patronus – Lily's Patronus, a doe. 'Always,' he said.