"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.
Dumbledore watched her fly away, and as her silvery glow faded he turned back to Snape, and his eyes were full of tears."After all this time?""Always," said Snape.