This is an amazing piece. Celestia and Luna/Nightmare Moon are portrayed beautifully; I love how it shows Luna's utter rage and jealousy of her sister and the grim determination of the Celestia to stop her beloved younger sister. Love the colors you used for both outfits, accenting the alignments of the two sisters and I feel especially Celestia's outfit is amazing in how it falls around her. Aside from the fact that it looks like a cutscene straight out of an RPG game(and by that, I mean that it looks in that kind of style.). Combined with all of the little details you put in, the eclipse in the background and the fact that Luna actually looks like she is bringing all of the power of the night to bare upon my favorite princess makes this such a great piece. If I were to change anything(nitpick time!), I wouldn't have given Luna a hood. I feel that it just looks out of place in such a grand battle, but that is such a small point that it really doesn't subtract from the picture on a whole. Thank you very much for what I consider one of or even my favorite piece of MLP fanart.
Finally some armor design that actually suits Nightmare moon characteristics I'm kinda sick seeing people make her over the top sexual or slutty Btw you kinda get 1 of the elements wrong there cuz I don't remember there's a green one Besides that, great job