A violent curse used to injure or maim an opponent
Blood spurted from Malfoy’s face and chest as though he had been slashed with an invisible sword.
Harry Potter and the Half-Blood Prince
By J.K. Rowling
Sectumsempra was always a speciality of Snape’s. I wish I could say I’d paid him back in kind, but it was all I could do to keep George on the broom after he was injured, he was losing so much blood