Wow, nice job. There are a few typos that you might want to fix, but otherwise, well done.
I think this is really great. It *sounds* like Narcissa - you really captured her. My only nitpick is near the end where you used "it's" instead of "its" (should be: "who even knows if it will survive if it caught its father's condition"), and I was surprised that Narcissa would ever mention Muggle characters, or deign to have read the books they come from. Other than those, great fic!
Very gloomy subject. Well written, but not fun to read.