Hey KP! Before I start, gotta say, love your stuff, I currently have 'See you Later' [link] set as my wallpaper, and I don't see it changing anytime soon!
Fan-mode aside, this is a really amazing and impacting piece that draws the viewer in even before they've seen past the thumbnail.
The main thing that I like about this picture is that it isn't exactly clear what is going on; when I first saw this picture (a couple of weeks ago), I was very much under the impression that Celestia was 'possessed' be Chrysalis and was making her transformation much like at the end of season 2. And since you marked it 'Season 3 spoiler', I assumed this was what was going to happen in the episode.
What I'm getting at is that it creates real mystery. You're seeing Celestia doing something that we've never see her do. If we assume that she was using her own magic (which she was), it comes as a surprise because we've never seen her really do anything other than a bit of levitation here and there. It really makes you look at it for ages trying to figure out what's going on.
I'm not that amazing of an artist so I don't usually critique artists of your calibre, but I can definitely see some things that don't seem right in the picture.
First off, I'll commend you on your good use of a consistent light-source (from where the viewer is) and adding shadows accordingly. I can see a few places where some shadows need to be added, for instance, her little bit of blue mane on the left; there should be a shadow on her jaw, her crown should make a shadow on her mane, and for a bit of advice, even things like smoke cast a light shadow. Doing this can make the smoke/magic dust seem more realistic and like it's actually there.
Second thing is that there is a surprising lack of reflected light on the subject (Celestia). Her green eye is obviously acting as it's own light source (indicated by the fact that it's glowing), but it doesn't seem like the light is hitting anything but the direct face around it. In reality, the dark space inside the mane should be lit green, and so should the edges of closer bits of mane. Where you've got white highlights, there should also be green reflections. Because that's all highlights are; reflected light from the light source. Reflected light is extremely important, because it can really be what makes your illustration 'pop' and seem more life-like. And I know that life-like is not what you're aiming for, but it can really emphasise your work.
I think I've covered everything that needs to be, and I hope I have been helpful. I really can't wait to see some new works and potential wallpapers from you again, KP!
now pardin my spelling but i love the shadow work on this how her main looks the light and shadow comeing off her eyes this is in amaysing peace of work and i must say i have never seen anything like this and the hole consept of it is amaysing the feeling the emoshon is just wow iv seen alot of thees dark celestia picters today but this one is vairy fantastic and i wonder what program you used to make this if thair evin whas one i think that i would like to see more of your art in the time you are on da.
"Now, Twilight, I'm going to give you a test, and if you pass, you get to move on to the next level of your studies (mwahahaha...). Here, you'll need to know this dark magic spell to pass the test. All you have to do is channel your anger and hate, like this! ...Yes, good, my young apprentice! Let the darkness flow through your heart!"
... You have just become my favorite person in the world for the rest of the day. Love this SO MUCH. That dark side of Celestia we saw in the episode really gave me the chills and this picture brings that feeling right back without any hesitation. It is going in my faves and probably on my desktop as the wallpaper. ^.^ Also, gonna link this in my brony discussion group.