Oh, wow, this is astounding. The relationship is lovely the sex scene realistic and revealing and hot, but god, what I really love about this story are the characters. The way you take the Snape from canon - the simplified Snape who was in love with Lily all his life and loyal to Dumbledore - and give him all the layers that make him such a fascinating character to so many fans. His analytical mind, his dislike of Dumbledore only eclipsed by his hatred of Voldemort, his Slytherin nature in that he, too, sees in people the way he can use them, but his kindness as well. His reverence for Sybill is beautiful, and how apt that he sees through the shawls and the wide eyes to who she really is.
And this is the best Sybill I have ever read. By now he had seen Sybill reading tea leaves with a better-than-average degree of accuracy. It was always small stuff the patterns foretold, and its very triviality led her to see it as unimportant. THIS. This is so perfect I can hardly put it into words. I have so long thought that Sybill's lack as a teacher and a divinator wasn't that she couldn't see things (she proves in her predictions at the beginning of PoA that she can), but that she gets so bogged down in seeing the big things and interpreting them in large ways that she misses the more subtle meanings. And you put it so eloquently here, you explore it so wonderfully, along with how she came to be that way, that it takes my breath away.