This spell binds a wizard to a promise, who will die if they break it
If you are there to protect him ... Severus, will you swear it? Will you make the Unbreakable Vow?
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...
Harry Potter and the Half-Blood Prince
By J.K. Rowling
I am trying to help you. I swore to your mother I would protect you. I made the Unbreakable Vow, Draco –