It's unbreakable

Harry tells Ron about what he overheard

Extract from Harry Potter and the Half-Blood Prince

by J.K. Rowling

‘So Snape was offering to help him? He was definitely offering to help him?’

‘If you ask that once more,’ said Harry, ‘I’m going to stick this sprout –’

‘I’m only checking!’ said Ron. They were standing alone at The Burrow’s kitchen sink, peeling a mountain of sprouts for Mrs Weasley. Snow was drifting past the window in front of them.

‘Yes, Snape was offering to help him!’ said Harry. ‘He said he’d promised Malfoy’s mother to protect him, that he’d made an Unbreakable Oath or something –’

‘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.

What happens if you break it, then?’

‘You die,’ said Ron simply.

‘Fred and George tried to get me to make one when I was about five. I nearly did, too, I was holding hands with Fred and everything when Dad found us. He went mental,’ said Ron, with a reminiscent gleam in his eyes.

‘Only time I’ve ever seen Dad as angry as Mum. Fred reckons his left buttock has never been the same since.’

Harry Potter and the Half-Blood Prince

By J.K. Rowling