Stops the bleeding and closes the wounds made by the Sectumsempra curse (HBP24).
References from the canon
- Harry used Sectumsempra on Draco in Moaning Myrtle's restroom during 6th year. He had seen the spell in the Half-Blood Prince's potion book, listed "for enemies," and indeed, it made Malfoy bleed as if "slashed by an invisible sword" (HBP24). Snape entered and quickly began tracing the wounds with his wand to close them while chanting the song-like countercurse, which had to be performed three times before the bleeding stopped (HBP24). Even after using the countercurse, Snape said that Draco would possibly have scars, but using the herb dittany would help.
- Harry wondered if something similar to Snape's countercurse would work on Bill Weasley's werewolf bites from Fenrir Greyback, but Madam Pomfrey said the wounds were too cursed to be reversed (HBP29).
- Mrs. Weasley performed the countercurse to stop the bleeding of her son George after Snape accidentally removed his ear with Sectumsempra during the Battle of the Seven Potters. However she told Harry she could not replace his ear because Sectumsempra was Dark Magic (DH5).
- When Harry accidentally cut his finger on a broken piece of the two-way mirror Sirius gave him, he couldn't mend his own wound and regretted that "serious flaw in his magical education" (DH2).
Many readers assume that Snape knew the countercurse for Sectumsempra because he invented the spell, but that is not clear from the canon. As a boy, Snape merely wrote the name of the spell in the margin of his potions book, and there is no canon about where the spell came from originally. However, it's possible that Snape (as the young Half-Blood Prince) may have invented or refined the countercurse since he performed it so well after Harry and Draco's duel. It's also possible that Snape taught the countercurse to Molly Weasley and other members of the Order of the Phoenix.