I don't rec many fics - actually I don't read too many these days. For those Snape-lovers out there who are tearing their hair out over "Was he?" "Wasn't he?", this short one-off is an excellent read. I can't recommend it highly enough. A missing scene from the book between Snape and Dumbledore and it reads totally in character and way better written than canon.
An Unbreakable Promise by Azazello A snippet . . . . Snape said, “It’s an insane idea, and I was insane ever to listen to you. I’ve followed your lead for almost seventeen years; obedient to your wishes, regardless of what they meant for me, but this is the end. You cannot seriously ask me to-"
“It may not come to it, as you well know, Severus. But I very much doubt there is a way to escape the consequences of your Vow with Mrs Malfoy, should you fail to fulfil it-"
Go read it. You won't regret it.