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 and inflicting near-horrific scarring and internal bodily injuries, to the point they are disfigured or possibly killed. 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, most likely also invented by Snape, but all that is known is that it is "an incantation that sounded almost like song". 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 bullied by James Potter, Snape used a much more limited version of the spell while fighting him that only gave him one deep gash on his cheek.
 * 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 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 permanently lost an ear due to this injury.

Etymology
Sectumsempra derives from the Latin sectum, "having been cut", and possibly semper, "always".

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