Here's a quick drawing I did tonight... Celestia is taking longer than expected because of her dumb wing >.>
Who knew that drawing a realistic looking wing/feathers would be difficult? Oh, I might have known that before hand...
I think this took about 45 minutes to do? An hour tops.
Edit: Woot! Revisions! Ok, so I fixed her legs, her ear and her back. Fixed her hair. Made her horn glow, as well as her cutie mark and put in a different dagger. Now for the Q&A! And FYI, there are possible spoilers coming up so if you haven't read past chapter 16, don't continue reading.
Why is her cutie mark glowing?
This is my interpretation of magic. According to the story, the weapons the four ponies make (NMM, Celestia, Luna and Twilight) are made of arcane energy. In other words, this isn't ordinary magic. To me, it is a lot more powerful than anything we've ever seen. To be fair, let's look at it this way. The most powerful ponies ever made a giant sword. Luna, with her smaller size as power, made two small swords. Twilight, probably the most powerful unicorn pony to exist, made a dagger. Since her cutie mark is related to magic, I felt like some of her magic 'strength' flows from here as well.
Why is her horn glowing white?
Same as before, except her horn is her 'magic output.' So from the increase power of the magic she is using, her horn gets this weird glow.
What's up with the dagger?
Ok, one thing I did not agree with in this scene is that Twilight learns a new spell and is able to master it immediately. If we assume that Nexus taught her that spell, well then, he must have read up on it because I'm sure Celestia wouldn't teach anyone a spell that it's sole purpose was to injure alicorns. And if so, why did he not use it during the prelude? My reasoning is that this is an extremely powerful spell requiring a lot of magic. This is why Twilight was able to ATTEMPT the spell. In my opinion, Twilight should have been able to either conjure up the dagger but have a hard time controlling it OR the dagger must have not been a beautiful, clean weapon. Although I do say that I prefer the first description, since she was able to injure NMM, I guess I'll just draw a damaged dagger... Oh well... I mean she did miss... But in my head, that scene should have played completely different.