“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.”

Harry Potter and the Half-Blood Prince – J.K. Rowling