This is not good fanfiction, simply because it transcends fanfiction all together, and becomes something greater than intended. You're only fault; (only major fault I should say) is that you don't really seem to be writing about the Harry Potter universe. What you have written is so powerful and deeply moving in an emotional level, and transcends fanfiction. You do this with your description which is hauntingly lovely, yet never overdone. You promise this in the very first lines:
Insanity is ugly. Especially when it's the good guys that are insane.
They circled 'round and 'round the Death Eaters, as vultures descend upon their prey. The sky was an odd shade of iron, where the horizon glowed brightly but the sky was grey.

The first sentence is a great hook crafted almost in the tradition of an essay, but then with surprisingly you switch to an almost Victorian tone seamlessly.
Really to me, the beginning evokes Lord of the Flies. The way you describe them all as children even though they are not children, and in your story do not even act as children. You are not tied down by characterization and while others may decry this as not cannon, or other such critisms, I think that this is what really allows the piece to soar. It’s your own style and your own story and I think very easily could be translated into your own original fanfiction.
The emotional depth of your dialogue is also what makes this piece shine. It is incredibly rare to find an author, original fiction or otherwise that can write emotional dialogue without being trite, but you do it beautifully.
However, I do have a small critism about the end. There is very little comedy in your piece and while I understand you were trying to evoke a bittersweet imagery, with the comedy and tragedy or perhaps a kind of harlequin morbid tragedy, it doesn’t really work as the only mocking as been in the description, and especially because you haven’t mentioned anything remotely “happy” since perhaps the beginning of the story or not at all. This also segways into my confusion about the title itself, and the meaning therin. I know what mood and the message you were trying to achieve, but perhaps if you streamline the themes more. It’s a lovely complicated piece but would be even better I think if instead of just trying to evoke a mood you also interweave a message. (There might have been one I just couldn’t see it.)
All in all fantastic job.
P.S This is going on my favorites list.

Author's Response: However, I do have a small critism about the end. There is very little comedy in your piece and while I understand you were trying to evoke a bittersweet imagery, with the comedy and tragedy or perhaps a kind of harlequin morbid tragedy, it doesn’t really work as the only mocking as been in the description, and especially because you haven’t mentioned anything remotely “happy” since perhaps the beginning of the story or not at all. There\'s no comedy in the story at all, as there isn\'t meant to be any. Comedy and Tragedy are merely references to the masks. The only \'comedy\', as it were, is the laughter of the masks, which reflect, to a degree, the insanity of the children. It\'s not meant to be bittersweet or have any sort of humourous twist, even with dark humour. Though I can see where the confusion would come from. I am glad you enjoyed it, though, as I was worried about how people would take a fairly dark fiction that deals with mental instability. Thank you for taking the time to even crit- it does mean a lot and it helps me grow as a writer. - Seren