U is for Unbreakable Vow

"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. (The Half-Blood Prince, 325-326)

The Unbreakable Vow is a mysterious spell that we only encounter once in the Harry Potter series.  Ron mentions that Fred and George tried to get him to make one when he was five, but luckily their dad was able to intervene before anything happened.



However, the Unbreakable Vow between Snape and Draco's mother actually happened, in another valiant effort by Snape to secure his trustworthiness with Voldemort's supporters.

Narcissa is very worried about the mission that Voldemort has given to her son: to kill Dumbledore.  She asks Snape to look after him, but Bellatrix keeps insinuating that Snape is not loyal enough to actually do anything.  Therefore, when Narcissa suggests the Unbreakable Vow, Snape agrees, to Bellatrix's shock.

"Will you, Severus, watch over my son, Draco, as he attempts to fulfill the Dark Lord's wishes?"

"I will," said Snape.

A thin tongue of brilliant flame issued from the wand and wound its way around their hands like a red-hot wire.

"And will you, to the best of your ability, protect him from harm?"

"I will," said Snape.

A second tongue of flame shot from the wand and interlinked with the first, making a fine, glowing chain.

"And, should it prove necessary...if it seems Draco will fail..." whispered Narcissa (Snape's hand twitched within hers, but he did not draw away), "will you carry out the deed that the Dark Lord has ordered Draco to perform?"

There was a moment's silence.  Bellatrix watched, her wand upon their clasped hands, her eyes wide.

"I will," said Snape.

Bellatrix's astounded face glowed red in the blaze of a third tongue of flame, which shot from the wand, twisted with the others, and bound itself thickly around their clasped hands, like a rope, like a fiery snake. (36-37) 

I find this spell so interesting and, horrible though this may sound, I wish we had seen someone break an Unbreakable Vow in the book.  I don't understand exactly how this magic is supposed to work.... How exactly does the spell determine what the best of Snape's ability to protect Draco might be?  And how does one die from breaking the Vow? Do they suddenly drop dead once a condition is broken?

Tomorrow will be my 21st Harry Potter themed A to Z posts! It's V for Veritaserum!