Sectumsempra is curse invented by the Half-Blood Prince. It acts like an invisible sword, slashing the victim with deep wounds, causing them to bleed heavily.
The curse is written in Harry's copy of Advanced Potion Making by its previous owner, the Half-Blood Prince. The wounds are not immediately fatal to the victim, although they are at risk of dying from the heavy blood loss. A counter-curse exists to the spell, a song-like incantation, which Professor Snape uses on Draco Malfoy after Harry uses Sectumsempra during their duel, and dittany can be used to prevent any scarring.
