I adored this. I found the first half incredibly difficult to read as I'm a chronic agoraphobic myself and Severus' fears and anxiety, his desperation and the feeling of fighting an faceless enemy all rang so truly to me. Why bother getting up and dressed? No-one will see you, you're not going anywhere...and of course, the never admitted acceptance that comes late at night, when you can't sleep; that you can live like this, really, because it's become all you know and you don't know how to be any other way. And I loved how true you stayed to that.
I resonated so strongly with Severus in this, it felt like reading back times in the last five years that I've been at my worst. I always worry when agoraphobia is mentioned in fic, it hits me too hard, especially if it's not dealt with sensitively and with a truth in it...but it was by you and I couldn't not read it. I'm glad I did. This was just perfect.
I began to suspect all was not clear when Severus fainted when he showed Harry his symptoms - no agoraphobic faints, as much as we fear it. The hyperventilation makes it impossible to pass out - but that's rather specialised knowledge, so I had a little bit of an advantage, lol.
I must admit, my mind turned totally away from all of that as the mystery began to unfold. I adore a good old mystery (it's because I'm English, I'm sure...) and this really lived up to that. It was so complicated and thought provoking within becoming bogged down by unnecessary twists and turns, it was long and well-developed, but I didn't feel like I'd been reading forever just to get to the end. Every scene was important to the story and it barely seemed that a good hour had passed.
I adore Harry and Severus' relationship in this. Not every love affair is straight out of Russian literature, lol. The sense of quiet, of need, of acceptance and love...it was breathtaking and more poignant for it.
I'm rambling, so I'll stop. ;-)
Just know that you really touched me deeply with this, and for that, I offer what little I can - my heartfelt appreciation and thanks.