Bonder and Unbreakable Vow

[fanofminerva]

No: HPFGUIDX 136726

"Certainly, Narcissa, I shall make the Unbreakable Vow," he said 
quietly.  "Perhaps your sister will consent to being our Bonder."
   Bellatrix's mouth fell open.  Snape lowered himself so that he was 
kneeling opposite Narcissa.  Beneath Bellatrix's astonished gaze, they 
grasped their right hands.  (HBP, US pg.36)


Upon my first reading, I assumed her astonishment was because she was 
convinced Snape was working for Dumbledore and was suprprised when he 
agreed to the unbreakable Vow.  Now that I have re-read it a few 
times, I still think this is the primary reason for her astonishment.  

However, I began to wonder what role the Bonder plays in the vow.  Are 
there any consequences to the Bonder if the vow is broken?

Julie