Sectumsempra

"Blood spurted from Malfoy's face and chest as though he had been slashed with an invisible sword. He staggered backward and collapsed onto the waterlogged floor with a great splash, his wand falling from his limp right hand. Slipping and staggering, Harry got to his feet and plunged toward Malfoy, whose face was now shining scarlet, his white hands scrabbling at his blood-soaked chest."

- Description

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.

Overview
A rather dangerous curse, when the incantation is uttered its effect is the equivalent of an invisible sword; it is used to slash the victim from a distance, causing rather deep wounds. The slash follows the user's wand movements. Due to the depths of the cut, the victims run the risk of dying from blood loss if treatment is not applied in time, if the wounds are not instantly fatal.

Counter and Treatment
Wounds inflicted by this spell can be cured by the song-like incantation Vulnera Sanentur; the first usage eases the blood flow, the second causes the wounds to knit and the third removes the worst effects of the curse. The victim would still require medical treatment, and if dittany is applied immediately, scarring can still be avoided.

Though the aforementioned healing spell may heal the wounds and dittany may prevent scars, any body parts that have been severed by this curse cannot be grown back (a trait of dark magic's permanent effects), as demonstrated by George Weasley when he lost his left ear to the spell.

Practitioners

 * Harry Potter
 * Severus Snape

History of usage
The curse was invented by Severus Snape during his time as a student at Hogwarts, in retaliation against his enemies. He recorded it in his textbook and used it enough to make Remus Lupin recognise it as one of his signature spells.

Many years later, in the 1996–1997 school year, Harry Potter came into the possession of Snape's textbook, and learned the incantation. Without knowing its effects at the time and believing that the "Half-Blood Prince" (Snape's nickname signed into the book) merely copied it as a note of reference, Harry became interested in trying it, thinking it would have some hilarious effects as with Levicorpus and other spells found within the same book. He considered using it against Cormac McLaggen for annoying him, though it was not until he confronted Draco Malfoy in Moaning Myrtle's Bathroom that he utilised it, mortally wounding Malfoy. Snape saved Draco, and having realised that Harry got a hold of the old textbook, he punished Harry with a multitude of detentions for nearly killing Malfoy. Harry, despite disliking Malfoy, did not truly want to harm Malfoy to such an extent, and was both horrified and guilt-ridden by using the curse against him. Professor McGonagall considered Harry to be lucky not to have been expelled for such an act, and she full-heartedly supported the detentions, while Pansy Parkinson wasted no time in vilifying Harry.

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. Later the same night, when Snape killed Dumbledore, Harry attempted this curse against him, only for Snape to negate it and angrily confront Harry for using it against him, revealing that he is the inventor of the said curse.

"You dare use my own spells against me, Potter? It was I who invented them - I, the Half-Blood Prince! And you'd turn my inventions on me, like your filthy father, would you? I don't think so... no!"

- Professor Snape to Harry during the Battle of the Astronomy Tower

The next year, during the Battle of the Seven Potters, Snape attempted to use this curse to sever the hand of a fellow Death Eater, but due to a slip, he accidentally cut off George Weasley's left ear instead. The ear could not be repaired by any means due to the nature of Dark Arts.

Etymology
Sectumsempra derives from the Latin sectum, "having been cut" (comparable to the English word "section" or "segmented"), and possibly semper, "always." It can be translated as "always cutting" or "sever forever", the latter being a pun on Snape's first name, Severus, as well as indicating that severed body parts cannot be regrown.

Behind the scenes

 * Sectumsempra, like Levicorpus, most likely escaped Snape's secrecy as well, seeing how Remus Lupin recognises it during the Battle of the Seven Potters.
 * In the films, Sectumsempra appears as a small flash of white light which causes a cut when it strikes the target.
 * 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.
 * This curse's effect is similar to the Severing Charm, except it is used for more malicious intent.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter: Spells
 * LEGO Harry Potter: Years 5-7