This is an interesting short story. i think you've captured a certain quality about Luna which most people miss, which is that she's not just a dippy and dreamy girl who spouts nonsense, but someone who's intelligent and wants to fight. So I enjoyed the exchange with Neville at the beginning of the story, as well as the interweaving of all the magical beasts she sees and believes in. It was a credible Luna, although at times I think her inner voice sounded a little mature for her. (Mind you she's sixteen/seventeen so perhaps not.)
I have one or two quibbles, though. The part about taking the Muggle-bornm students away made it sound as if they'd actually taken them away from Hogwarts, but they'd actually banned them from going there in the first place, so all the Muggle-borns stayed at home or went on the run. That could just be me misinterpreting your meaning, though. The other thing is Ginny teaching Luna how to Apparate. She's barely sixteen herself and wouldn't have had official lessons. Okay, so Fred and George might have taught her the rudiments, but I very much doubt she'd have been capable of Apparating herself, let alone instructing Luna. That's a minor quibble, though. I did like the fact that Ginny had refused to pack her wand. Very in character.
I also appreciated the fact that you gave the Death Eaters very different 'voices'. One definitely sounded much rougher than the other. Having read a lot of fanfiction where writers fail to differentiate between their characters - especially the minor ones - I thought this was a very good touch on your part. Not all Death Eaters spoke like Lucius or Snape, after all. We see that in the speech patterns of Amycus, so well done for that.
This is an interesting description of a little moment in time, when Luna is captured by the Death Eaters. It is written in the first person, from Luna's point of view. The short sentence structure evokes her unique thought processes. She jumps, in her mind, from topic to topic, and thoughts of the imaginary creatures invented by her father intrude on her attempts to understand what is going on around her.
The first conversational section, between Neville and Luna, seems unsubstantial to me. I'm not sure what it contributes to the main theme of the story. The collection of short paragraphs that follow are a good description of the changes and deterioration of the environment in the school and in society in general as Voldemort rises in power. Then the last of these short paragraphs, where Lune speaks of the Crumple-Horned Snorkack, reminds us again that her grasp of the situation (and therefore her reaction to it) is still distorted by her belief in her father's "crazy theories".
In the end, she is too distracted to protect herself, to Ginny's great frustration, when the Death Eaters seize her and take her away. She asks, "How do you Disapparate?" but I do seem to remember that somewhere in the books Luna says that her father taught her to Apparate when she was younger than the usual age for Hogwarts students to learn the skill. (Maybe I disremember.)
Writing Luna must be a challenge. (I have never tried it.) It would be a mistake to stereotype her as a ridiculous figure. She was not a stupid or crazy person, despite her adherence to her father's beliefs. She was a successful Hogwarts student, but with a unique outlook on life and a remarkably even temperament. The author has captured her personality well.
I LOVED Luna's thoughts about the nargles! (and wrackspurts, etc.) Although there were a couple of typos scattered through the story, and you should expand some parts, I generally liked it.
Short but tells the story. Very believable Luna.