Sectumsempra

Sectumsempra is one of the few spells in the books known to have been invented by a named character — Severus Snape, during his student years at Hogwarts. The incantation derives from the Latin sectum (cut) and semper (always): roughly "ever-cutting." Cast at a target, it produces deep, sword-like slashes wherever it lands, opening serious wounds that bleed dangerously and resist conventional healing.

Harry meets the curse the way most readers do: by accident. In the sixth-year Potions classroom drama, Harry has been working from Snape's old textbook — the famed Half-Blood Prince's copy — and has copied the marginal note next to Sectumsempra: "For Enemies." When he confronts a sobbing Draco Malfoy in the boys' bathroom and the duel turns ugly, Harry cries the incantation without knowing what it does. Draco crashes to the wet floor with cuts opening across his face and chest, blood spreading in the water. Snape arrives just in time to save him, kneeling over Draco and singing the counter-curse in a low chant. The scene is the moment Harry realizes the textbook he has been cheating off all year belongs to Snape himself.

Sectumsempra's counter, Vulnera Sanentur, has to be sung — three repetitions in succession — and works only if cast quickly enough. Without it, the wounds bleed badly and resist standard healing magic. George Weasley loses an ear in the Battle of the Seven Potters when one of Snape's cast Sectumsempras glances off him in midair. There is no scar tissue replacement; the ear is simply gone, and the loss confirms Snape's authorship of the curse to anyone who knew where to look.

Despite Snape's lifelong loyalty to the Order, Sectumsempra is a curse with no clean defensive use. It is meant to wound. The fact that Snape both invented it as a student and later wrote down its counter-curse — the only one of the wartime healing songs the books name explicitly — captures a great deal of who he was: brilliant, dangerous, and doubled.