"Apparently I underestimated you, Potter. Who would have thought you knew such Dark Magic? Who taught you that spell?"
-- Professor Snape to Harry (HBP24)
A violent, Dark Magic spell which cuts and severely injures its target. Without immediate healing, the victim may die.
History and Notes
This spell was invented by the Half-Blood Prince c. 1970s and written in the margin of his potions textbook (HBP21)
References from the canon
A spell invented by the Half-Blood Prince "for enemies" (HBP21)
Considered to be Dark Magic; anything severed from a target's body by this spell cannot be grown back by magic, according to Molly Weasley (DH5).
Harry cast this on Draco. Snape said afterward that there might be a certain amount of scarring, but that immediate treatment with dittany might prevent that (HBP24)
Apparently the nonverbal spell used by Snape on James Potter that resulted in a gash across James' face during their confrontation just after their Defence Against the Dark Arts O.W.L, as seen by Harry in the Pensieve (OP28), judging from Snape's remarks during his departure from Hogwarts (HBP28).
Harry used this on Inferi in the cave, but it didn't do much good (HBP26)
Harry tried and failed to cast this on Snape during Snape's departure from Hogwarts (HBP28).
"sectus" L. past participle of "seco", to cut + "sempra" L. always
From the Web
Sectumsempra on the Harry Potter Wiki
Why wasn't Harry punished after he used Sectumsempra on Malfoy? from the SciFi Stack Exchange