QUOTE : What Narcissa has done for him with the Unbreakable Vow is beyond measure. Until this vow, Snape has never been the most important person to anyone in his adult life. But now someone's entire happiness depends upon his survival and success. In the moments when his courage fails him at the thought of killing Dumbledore, it will help to remember that he must stay strong for Narcissa. It strengthens him. He is no longer alone.