What happens when you break an Unbreakable Vow?

You pledge with your life. These are not called "Unbreakable" for nothing. You die. End of the story.

Ron explains it in HBP, Chapter 16. Same in the movie.

You die

You die *dramatic music*

yeah, i think ^

Yes you die that's why Bellatrix was so surprised that Severus was willing to do it. Because she never thought he would really help Draco but considering that if he didn't he would die I'm pretty sure she thought he wasn't able to put his own life in danger.

if you break an Unbreakable vow you die

u die

One word: Death.

You Die.

u die. end of story

In the book, when Ron explains it to Harry, he tells him that when he was little (like 5 or 6 years old) fred figglehorn and George tried to get him into an Unbreakable Vow and their dad caught them. Arthur had a kyniption, of course, and Ron says it was the only time in his life he ever saw his father freak out worse than his mother. LMMFAO.