A violent, Dark Magic spell which cuts and severely injures its target
The curse was invented by Severus Snape and recorded during his time as a student at Hogwarts, when he was known as “The Half-BloodPrince“[6]. He created it to retaliate against his enemies such as the Marauders[7], and he recorded it in his N.E.W.T.-level Potions textbook. He used it enough for Remus Lupin to 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. Knowing nothing more than that it was “for enemies” 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. He expected humorous effects, as he’d found with Levicorpus and other spells in the book.[8]
Harry initially considered using it against Cormac McLaggen for annoying him, though it was not until he confronted Draco Malfoy in Sixth-floor boys’ bathroom that he tested it, seriously wounding Malfoy. Snape intervened to heal Draco, and having realised that Harry got a hold of his old textbook, he punished Harry with a multitude of detentions; though for fear of exposing himself as the curse’s inventor, Snape chose not to turn Harry in for greater punishment.
Despite disliking Malfoy, Harry did not want to harm Malfoy to such an extent, and he was both horrified and guilt-ridden for using the curse against him. Professor McGonagall considered Harry to be lucky not to have been expelled for such an act and full-heartedly supported the detentions, while Pansy Parkinson wasted no time in vilifying Harry far and wide; Ginny Weasley, however, stood up for Harry’s usage of the curse, as it saved him from being struck by Malfoy’s Cruciatus Curse.[2]
During Harry and Dumbledore‘s hunt for a Horcrux, Harry accidentally attracted the attention of an army of Inferi. In an attempt to stop them, one spell Harry used was Sectumsempra, but it merely slashed their unfeeling flesh and did not seriously impede their approach.[9] Later that night, after Snape murdered Dumbledore, Harry attempted this curse against him, only for Snape to block it and angrily reveal to Harry that he was its inventor.[6]
1997–1998 school year
Remus Lupin: “He lost an ear.“Hermione Granger: “Lost an…“Remus Lupin: “Snape’s work.“— Lupin and Hermione discuss Snape’s handiwork with the curse[src]
The next year, during the Battle of the Seven Potters, Snape attempted to use the 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, leaving him guilt-ridden. The Order of the Phoenix was not able to regrow the ear, as curse wounds (especially those in the nature of the Dark Arts), are unable to be healed, and the counter curse was unknown. Due to the nature of the events at the time, the Order believed Snape acted on ill-intent, and their grudge against him intensified, until Snape revealed his misfiring, posthumously, via his memory to Harry.[5][10]
Our beloved Professor Snape 