A well-written, poignant story, with a believable version of Sybill that takes her so far beyond the shallowness of her canon persona. I like the way you use our knowledge of Dumbledore and of how the story ends to create such painful irony in Severus's thoughts about his future.
The scene in Sybill's room, where she and Severus discover and explore each other, is really effectively done: sweet and sad and pleasurable and not in the least pathetic. You bring Sybill into her own here.
that man-drowning sea of love and loss Excellent line.
One of my favorite things about Beholder stories is how often they make me completely believe a pairing that I would never have considered at all likely. This fic is another fine example.