An Unbreakable Vow?" said Ron, looking stunned. "Nah, he can’t have.... Are you sure?""Yes I’m sure," said Harry. "Why, what does it mean?""Well, you can’t break an Unbreakable Vow...""I’d worked that much out for myself, funnily enough.
J.K. Rowling, Harry Potter and the Half-Blood Prince
harry-potter, humor, malfoy, ron-weasley, snape, unbreakable-vow