Ho Ho Ho! It's Secret Santa Renny with your Christmas TGS surprise! It's... a maxed-out review for lucky ol' you!
Browsing your impressive author's page, I was immediately drawn to this story after reading the summary. I studied ballet very seriously when I was younger with hopes of becoming a professional myself so I'm very familiar with the ballet of "Swan Lake." I can definitely see how the ballet and the music in particular would spur you to write something of your own. It's definitely one of Tchaikovsky's best works.
I obviously wasn't expecting a direct retelling with Harry Potter characters inserted, but you surprised me with just how abstract this one-shot is. I don't think that this is a bad thing at all. It makes it enjoyable for both the reader who is familiar with the Swan Lake story but also accessible to someone who isn't. The "other pairing" ship is always interesting to me because unless it's very obvious from the description, you don't know exactly what you're getting (kind of like on Christmas morning!). It took me a little while to guess who the male was (ok, pretty much until you flat-out named him). I had Draco in mind for some reason, probably because of his marriage to Astoria in canon, but I'm glad you didn't go with that expected choice. Theodore Nott is someone we basically know nothing about besides his father being a Death Eater so he's a great blank canvas to work with. I also think having him be a Slytherin gives him that subtle "princely" feel like the male lead in Swan Lake. Obviously there were poorer people in Slytherin like Snape, but I think we all usually assume Slytherins were pure-bloods and loaded like the Malfoys.
Something I found interesting, and I don't know if you did this on purpose or if I'm reading too much into it, was that it seems like you combined the characters of Prince Sigfried and Von Rothbart, the evil sorcerer, in Theodore. It's convenient that Theodore happens to be a wizard since that ties into the ballet nicely. I guess the sorcerer's motivation for turning Odette into a swan is never actually given in the ballet, but I'd always assumed it was because he was in love with her and she rejected him (maybe "Ladyhawke" with Michelle Pfeiffer is influencing me subconsciously) and then he in turn made sure to prevent the prince from having Odette as well. In your story, Theodore obviously loves Daphne, or at least loves her appearance or the idea of her, and this borderline obsessive love both holds her captive, like Odette's swan curse, and eventually nearly destroys her. So in this story, your main male character is sort of both a protagonist and an antagonist at the same time. I found that kind of cool.
One thing I was surprised you didn't do was utilize Astoria a bit more. One of the more famous variations from the ballet is the dance of the Black Swan, so I was somehow expecting Astoria, who I imagine looks almost identical to Daphne from your descriptions like how Odette and Odile were nearly identical (and are usually danced by the same ballerina) to be used somehow has a tool to further drive her sister and Theodore apart. Maybe through Theodore cheating on Daphne with Astoria in his frustration with his seeming inability to make Daphne happy. But I also feel maybe in your story Astoria might be too young to fulfill this role of temptress? I got the impression from her idolization of her sister that Daphne was definitely a few years older than her (I have no clue how far apart they're supposed to be in canon.) I guess none of this matter since your interpretation of the ballet in the Harry Potter world is so creative and full of imagery rather than dialogue and what I'd consider "usual" storytelling for fan fiction, but I was a little jarred by the omission. It's probably my own fault for going into this with the ballet so familiar in my mind.
The imagery and description in this one-shot was just great. It was one of those one-shots that seems deceptively long when you're reading it compared to how long you feel it should be going by the word count. I could feel your purpose and thought while writing this, crafting every sentence carefully. I mean seriously, it's almost impossible for me to pick out examples since almost every act is full of incredibly creative and emotive descriptions. For instance this: "Wisps of fog furl over distant hills, creep up far-away valleys and gently curl around his feet. Isolation sleeps deeply amongst the landscape, resting in long grasses and dreaming in the woods. A murmur of a breeze rustles green leaves. It looks beautiful." I can almost see the scenery from the ballet, which is also abstract enough to let the audience inject some of their own imagination and the dancers their own interpretation of the ballet into the setting. Normally I'm not a huge fan of one-shots that are themselves broken up into numerous segments, but the way you did it was very fitting and not distracting at all. It's probably because the writing itself was so rich, like I said before, that it made each act feel longer than it was so they really could stand on their own within a single chapter.
The ending was surprising since it wasn't a double love suicide like the ballet but still very fitting for your characters. Daphne is still set free from the "curse" that had been slowly destroying her like Odette is in the end, but not by Theodore's/Sigfried's love. On the one hand, I feel so sorry for her that she didn't end up having the "happy ending" in the classical sense (though how happy it is for two people drowning themselves is debatable) but the feminist in me says, "Alright! You don't need that man who doesn't love you for YOU!"
All in all, this was a great piece. It makes someone like me who knows the ballet think about it a little harder. Did the prince love Odette herself, or just the beautiful swan-woman? Thanks for the interesting insight and the great read! Merry Christmas!