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. "No —" gasped Harry. 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."

- Harry Potter uses Sectumsempra on Draco Malfoy

Sectumsempra is a Dark curse invented by Severus Snape (under his alias of "The Half-Blood Prince") for use against his enemies, and it soon became one of his specialities. When uttered, its effect is the equivalent of an invisible sword, slashing the target repeatedly. It works best on living targets and has no noticeable effect on inanimate objects or dead ones, such as Inferi, other than slashing them pointlessly.

The curse also has a counter-curse, assumed to be Vulnera Sanentur, also invented by Snape. The first time it is used, the flow of blood eases up; the second time causes the wounds to knit; the third time removes the worst effects of the curse near-instantly. The victim would still require medical treatment, and if Dittany is applied immediately, scarring can be avoided.

Though the counter-curse can heal the wounds, and Dittany can prevent scars, any body parts that have been severed by this curse cannot be grown back, as seen when George Weasley lost his ear to it.

Known Uses

 * While attending Hogwarts School of Witchcraft and Wizardry and being harassed by James Potter, Snape used the spell while fighting him. James Potter, however, did not receive the full brunt of the attack due to Snape's lack of aim and nonverbal use; resulting in one deeSectumsempra_hp.JPG gash upon his cheek as opposed to potentially fatal wounds to his body.
 * In a duel with Draco Malfoy, Harry used the curse upon him when Malfoy was poised to use a Cruciatus Curse. The result was Malfoy almost dying, proving how deadly the curse can be.
 * While in the Horcrux cave, Harry used the spell on an army of Inferi left to protect the Horcrux. This was to no avail, as Inferi could neither feel the gashes inflicted upon them nor lose any blood from the cuts.
 * During the Battle of the Astronomy Tower, Harry attempted to use the spell against Snape as he fled the school with the Death Eaters. Snape, however, deflected the spell easily, and revealed that he was the spell's creator, the "Half-Blood Prince."
 * During the Battle over Little Whinging, Snape, attempting to stop a Death Eater from cursing Remus Lupin, aimed a Sectumsempra curse at him, but it accidentally struck George Weasley. George lost an ear due to his injury, which could not be healed because it was caused by a curse.

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.

Behind the scenes

 * Sectumsempra, like Levicorpus, probably escaped Snape's secrecy too, as Lupin mentions it as "always a speciality of Snape's." It may also be possible that Lupin did not know the verbal form of the spell before the Battle over Little Whinging, but heard it during the battle and only detected it by its effect he had seen before.
 * The spell was engineered by Snape to use on enemies. It is possible that while inventing it, Snape's intention was to use it on his enemies back then, the Marauders.
 * The spell seems to manifest itself as a small flash of light in the movies.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter: Spells

Notes and references
Sectumsempra Sektumsempra