OMG! I'm absolutely floored by this story. I love it! I was hooked from the beginning.
Your Severus is such a compelling character, and you layer him very well. He's more nuanced and intriguing than JKR ever wrote him, and I adore him. Harry is wonderful as well--with just enough Gryffindor brashness tempered with maturity.
I never imagined that I would like Tobias Snape. Not only do I like him, but my heart broke for Severus when his father died. It was, to me, one of the best scenes in the story.
The end, with Harry sacrificing himself for Snape, who was sacrificing himself for Harry, was perfect. Al coming to the Underworld after living a long life was just what the story needed. Bravo.